[polyml] local specifications in signatures

The syntax reference here http://www.csci.csusb.edu/dick/samples/ml.syntax.html#Specification suggests that I should be able to write

signature foo = sig local open CharVectorSlice in val baz : vector -> slice end end

(making use of "other_spec" in the syntax reference above)

But PolyML complains when it hits the "local"

Poly/ML 5.4.1 Release

use"foo.sig";

Error- in 'foo.sig', line 2. end expected but local was found

Am I wrong in thinking that that is a valid signature specification? Is there an official reference for the syntax of the language PolyML compiles?

Do I have to keep repeating LongStructureName.type_i_want in my specifications? Or maybe it is typical to just export types liberally (because I could do type v = CharVectorSlice.vector at the top of that signature, etc.)