Handle ref as pattern and expression form in parsing/
Handle := as an infix.  Test with conversion to MiniML$dec type.
Correct lexer so that ',' is a "single char token" and '~' is not.
Correct lexer's treatment of underscore.

It is another "single character token".
Define stringDecs of type : string -> MiniML$dec list option
Handle the underscore pattern.

Translate to an ast-pattern-variable called "_".