Indeed, expressions such as "3.0-42.5" caused a syntax error, as the unary minus
of real constant was in hard in the lexer.  "3.0- 42.5" worked, but still.

The problem then was that LicEvalConst refused to eval "- 3.14", that was
parsed directly as a constant, and that is now a call to the unary minus.
I've added specific code to handle this case.