pic2plot: diagnosing then versus and

[Ron Burk]

pic2plot would be much more pleasant to use
if it diagnosed common syntax errors in human-understandable
terms. The lack of explanatory errors is particularly
frustrating because much of the pic grammar is
English-like, leading to dismay when correct English
sentences produce only "syntax error near `x'".

One common problem is that English lets you
use 'then' and 'and' interchangeably to connect
portions of a path you are describing, while
pic does not. I tried to make the grammar diagnose
this error by changing gram.yy.

First, I added this to the grammar:

then_not_and:
    THEN
    | AND
        {
          lex_error("unexpected `and' -- did you mean `then'?");
          YYABORT;
        }

Then, I went to the non-terminal "object_spec" and replaced
    | object_spec THEN
with
    | object_spec then_not_and