Are the alternative spellings described below
allowed even when <iso646.h> is not included?
If so, this is an incompatible change to the existing C Standard.
Is that what was intended?

clive@x.co.uk (Clive Feather) writes:

 2) Add the following as alternate spellings of various tokens:

     New    Old   Old (using trigraphs)
     ----------------
     <:     [     ??(
     :>     ]     ??)
     <%     {     ??<
     %>     }     ??>
     %:     #     ??=
     %:%:   ##    ??=??=

 The spelling of a token does not affect its meaning (so [ can be
 balanced by :> ).