[polyml] multiple "where type"

Hi, does PolyML support the syntax

<sig> where type t = foo and t' = bar

This is not valid signature expression syntax, however the following is.

<sig> where type t = foo and type t' = bar ^^^^

You also have to be careful, because the following is valid (although it is only accepted by MLton and Poly/ML -- the ML Kit, Moscow ML, and SML/NJ reject it).

signature S = sig type t end where type t = int and S' = REAL

The indentation is intended to make clear that "and S' = REAL" is part of the signature binding, not the signature expression, using the productions

<sigdec> ::= signature <sigbind> <sigbind> ::= <sigid> = <sigexp> [and <sigbind>]

This also explains why the "type" above is not redundant.