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I think it would be a good idea to add a std::hash specialization for
std::nullptr_t:

namespace std
{
template <>
struct hash<nullptr_t>
{
    size_t operator()(nullptr_t) const noexcept
    {
        return 0;
    }
};
}

One case where it would be useful:
I have a variant class template (tagged union) that I have a hash
specialization for:
template <typename ...Types>
class variant<Types...>;

namespace std
{
template <typename ...Types>
struct hash<variant<Types...>> final
{
    size_t operator()(const variant<Types...> &value) const noexcept
    {
        // pseudo code equivalent:
        switch(value.typeIndex)
        {
        case variant<Types...>::typeIndexFor<void>: // for empty variant
            return 0;
        case variant<Types...>::typeIndexFor<Type1>:
            return value.typeIndex + hash<Type1>()(value.as<Type1>());
        case variant<Types...>::typeIndexFor<Type2>:
            return value.typeIndex + hash<Type2>()(value.as<Type2>());
        case variant<Types...>::typeIndexFor<Type3>:
            return value.typeIndex + hash<Type3>()(value.as<Type3>());
        ...
        }
    }
};
}

I am using the variant to represent a javascript type:
typedef variant<Object *, double, bool, std::nullptr_t, std::u16string,
Symbol *> JSValue;


having hash<nullptr_t> would avoid needing special case code to handle
having a null JSValue.


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<div dir=3D"ltr">I think it would be a good idea to add a std::hash special=
ization for std::nullptr_t:<div><br><div><div class=3D"prettyprint" style=
=3D"border: 1px solid rgb(187, 187, 187); word-wrap: break-word; background=
-color: rgb(250, 250, 250);"><code class=3D"prettyprint"><div class=3D"subp=
rettyprint"><font color=3D"#660066"><span style=3D"color: #008;" class=3D"s=
tyled-by-prettify">namespace</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"> std<br></span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"><br></span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>template</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> =
</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;&gt;</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><=
span style=3D"color: #008;" class=3D"styled-by-prettify">struct</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> hash</span><span styl=
e=3D"color: #080;" class=3D"styled-by-prettify">&lt;nullptr_t&gt;</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 size_t </span><sp=
an style=3D"color: #008;" class=3D"styled-by-prettify">operator</span><span=
 style=3D"color: #660;" class=3D"styled-by-prettify">()(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">nullptr_t</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" =
class=3D"styled-by-prettify">const</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> noexcept<br>=C2=A0 =C2=A0 </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><spa=
n style=3D"color: #008;" class=3D"styled-by-prettify">return</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #066;" class=3D"styled-by-prettify">0</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">}</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">};</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br><=
/span></font></div></code></div><br></div><div>One case where it would be u=
seful:<br></div><div><div>I have a variant class template (tagged union) th=
at I have a hash specialization for:<div class=3D"prettyprint" style=3D"bor=
der: 1px solid rgb(187, 187, 187); word-wrap: break-word; background-color:=
 rgb(250, 250, 250);"><code class=3D"prettyprint"><div class=3D"subprettypr=
int"><font color=3D"#660066"><span style=3D"color: #008;" class=3D"styled-b=
y-prettify">template</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>&lt;</span><span style=3D"color: #008;" class=3D"styled-by-prettify">typen=
ame</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span=
><span style=3D"color: #660;" class=3D"styled-by-prettify">...</span><span =
style=3D"color: #606;" class=3D"styled-by-prettify">Types</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #=
008;" class=3D"styled-by-prettify">class</span><span style=3D"color: #000;"=
 class=3D"styled-by-prettify"> variant</span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=
=3D"styled-by-prettify">Types</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">...&gt;;</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"><br><br></span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">namespace</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> std<br></span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"><br></span><span style=3D"color: #008;" class=3D"styled-by-prettify">tem=
plate</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </sp=
an><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><sp=
an style=3D"color: #008;" class=3D"styled-by-prettify">typename</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">...</span><span style=3D"color:=
 #606;" class=3D"styled-by-prettify">Types</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">&gt;</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"><br></span><span style=3D"color: #008;" class=3D"=
styled-by-prettify">struct</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"> hash</span><span style=3D"color: #660;" class=3D"styled-by=
-prettify">&lt;</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify">variant</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">&lt;</span><span style=3D"color: #606;" class=3D"styled-by-prettify">Type=
s</span><span style=3D"color: #660;" class=3D"styled-by-prettify">...&gt;&g=
t;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">final</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 size_t </span><span=
 style=3D"color: #008;" class=3D"styled-by-prettify">operator</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">()(</span><span style=3D=
"color: #008;" class=3D"styled-by-prettify">const</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"> variant</span><span style=3D"color:=
 #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;=
" class=3D"styled-by-prettify">Types</span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">...&gt;</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">&amp;</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify">value</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">)</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">const</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> noexcept<br>=C2=
=A0 =C2=A0 </span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>{</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #800;" class=3D"style=
d-by-prettify">// pseudo code equivalent:</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span=
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le=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify">value</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">.</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify">typeIndex</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">)</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color=
: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" =
class=3D"styled-by-prettify"><br></span></font><font color=3D"#660066"><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify">=C2=A0 =C2=A0 =C2=A0 =
=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">cas=
e</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><=
/font><span style=3D"color: #000;" class=3D"styled-by-prettify">variant</sp=
an><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><sp=
an style=3D"color: #606;" class=3D"styled-by-prettify">Types</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">...&gt;::</span><span sty=
le=3D"color: #000;" class=3D"styled-by-prettify">t</span><font color=3D"#66=
0066"><span style=3D"color: #000;" class=3D"styled-by-prettify">ypeIndexFor=
</span><span style=3D"color: #080;" class=3D"styled-by-prettify">&lt;void&g=
t;</span><span style=3D"color: #660;" class=3D"styled-by-prettify">:</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span sty=
le=3D"color: #800;" class=3D"styled-by-prettify">// for empty variant</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=
=3D"styled-by-prettify">return</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> </span><span style=3D"color: #066;" class=3D"styled-by=
-prettify">0</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></s=
pan></font><font color=3D"#660066"><span style=3D"color: #000;" class=3D"st=
yled-by-prettify">=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: =
#008;" class=3D"styled-by-prettify">case</span><span style=3D"color: #000;"=
 class=3D"styled-by-prettify"> </span></font><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">variant</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=3D"s=
tyled-by-prettify">Types</span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">...&gt;::</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify">t</span><font color=3D"#660066"><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">ypeIndexFor</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=
=3D"styled-by-prettify">Type1</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">&gt;:</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"><br></span></font><font color=3D"#660066"><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify">=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">ret=
urn</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> value<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">.</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify">typeIndex </span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">+</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> hash</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: =
#606;" class=3D"styled-by-prettify">Type1</span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">&gt;()(</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">value</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">.</span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">as</span><span style=3D"color: #660;" class=3D"styled-by-pre=
ttify">&lt;</span><span style=3D"color: #606;" class=3D"styled-by-prettify"=
>Type1</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;=
());</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></=
span></font><font color=3D"#660066"><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify">=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color:=
 #008;" class=3D"styled-by-prettify">case</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"> </span></font><span style=3D"color: #000;" =
class=3D"styled-by-prettify">variant</span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=3D"=
styled-by-prettify">Types</span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">...&gt;::</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify">t</span><font color=3D"#660066"><span style=3D"color: #000;" =
class=3D"styled-by-prettify">ypeIndexFor</span><span style=3D"color: #660;"=
 class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=
=3D"styled-by-prettify">Type2</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">&gt;:</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span st=
yle=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> value</span><span style=3D"=
color: #660;" class=3D"styled-by-prettify">.</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify">typeIndex </span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">+</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> hash</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=3D"styled=
-by-prettify">Type2</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">&gt;()(</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify">value</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>.</span><span style=3D"color: #008;" class=3D"styled-by-prettify">as</span=
><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><span=
 style=3D"color: #606;" class=3D"styled-by-prettify">Type2</span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">&gt;());</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span></font><font colo=
r=3D"#660066"><span style=3D"color: #000;" class=3D"styled-by-prettify">=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"style=
d-by-prettify">case</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span></font><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify">variant</span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">&lt;</span><span style=3D"color: #606;" class=3D"styled-by-prettify">Ty=
pes</span><span style=3D"color: #660;" class=3D"styled-by-prettify">...&gt;=
::</span><span style=3D"color: #000;" class=3D"styled-by-prettify">t</span>=
<font color=3D"#660066"><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify">ypeIndexFor</span><span style=3D"color: #660;" class=3D"styled-by-pr=
ettify">&lt;</span><span style=3D"color: #606;" class=3D"styled-by-prettify=
">Type3</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt=
;:</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #008;" =
class=3D"styled-by-prettify">return</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"> value</span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">.</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify">typeIndex </span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">+</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"> hash</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&l=
t;</span><span style=3D"color: #606;" class=3D"styled-by-prettify">Type3</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;()(</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify">value</span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">.</span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">as</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #60=
6;" class=3D"styled-by-prettify">Type3</span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify">&gt;());</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 =C2=A0 =C2=A0 </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">...</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br></span></font><font color=3D"#=
660066"><span style=3D"color: #000;" class=3D"styled-by-prettify">=C2=A0 =
=C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #660;" class=3D"styled-by=
-prettify">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify=
"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #660;" class=3D"styled-by-=
prettify">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"=
><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify">};</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">}</span></font></di=
v></code></div><div><br></div><div>I am using the variant to represent a ja=
vascript type:<div class=3D"prettyprint" style=3D"border: 1px solid rgb(187=
, 187, 187); word-wrap: break-word; background-color: rgb(250, 250, 250);">=
<code class=3D"prettyprint"><div class=3D"subprettyprint"><font color=3D"#6=
60066"><span style=3D"color: #008;" class=3D"styled-by-prettify">typedef</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> variant</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><spa=
n style=3D"color: #606;" class=3D"styled-by-prettify">Object</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">*,</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=
=3D"styled-by-prettify">double</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">,</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify=
">bool</span><span style=3D"color: #660;" class=3D"styled-by-prettify">,</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> std</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">::</span><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify">nullptr_t</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> std</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">::</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify">u16string</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify"> </span><span style=3D"color: #606;" class=3D"styled-by-pret=
tify">Symbol</span><span style=3D"color: #000;" class=3D"styled-by-prettify=
"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">*&gt;</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><spa=
n style=3D"color: #606;" class=3D"styled-by-prettify">JSValue</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">;</span></font></div></c=
ode></div><br></div><div><br></div>having hash&lt;nullptr_t&gt; would avoid=
 needing special case code to handle having a null JSValue.<br><br><br></di=
v></div></div></div>

<p></p>

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