Tests if a one-character string is a symbol other than bracket or separator.
The call issymb s tests whether the first character of string s (normally
it is the only character) is ``symbolic''. This means that it is one of the
usual ASCII characters but is not alphanumeric, not an underscore or prime
character, and is also not one of the two separators `,' or `;' nor any
bracket, parenthesis or curly brace. More explicitly, the set of symbolic