Added syntax for structure array references, but it's actually broken.  Checking in the code so that I don't lose this idea.
- parses a subset of struct references.
- parses Matlab block comments
OMG, I think I have the matrix transpose operator working.  (A single quote! A single quote when Matlab strings are delimited by single quotes! What is wrong with those people?)
1. Parse commands like "hold on".
2. Forgot to put "for" statements into the operand regexp.
3. Matrix transpose has to be checked earlier in the operand regexp.
Fixed more bugs in handling the transpose operator.  This is still not perfect; I can't get it to parse case valid_47, which has a parenthesized expression followed by a transpose operator.  I'm leaving it for now, because I'm out of ideas.