/*
=for apidoc Am|OP *|newWHENOP|OP *cond|OP *block
-Constructs, checks, and returns an op tree expressing a C<when> block.
+Constructs, checks, and returns an op tree expressing a C<whereso> block.
C<cond> supplies the test expression, and C<block> supplies the block
that will be executed if the test evaluates to true; they are consumed
by this function and become part of the constructed op tree.