improvement for error message.

[Chris Dams]

On Tue, 16 Sep 2003, Chris Dams wrote:

> In the function ex::subs(const ex & e, unsigned options) if the first
> argument is neither a relation_equal nor a list, the error message
> "basic::subs(ex): argument must be a list" gets thrown. I think it would
> be a good idea to change this into "basic::subs(ex): argument must be a
> relation_equal or list". Imagine me trying to type a relation_equal,
> getting some parentheses wrong and ending up with something that is not a
> relation_equal nor a list. In that case the former error message makes me
> think "when did it happen that I cannot use a relation anymore as the
> argument of a substitution". The latter error message might put me on the
> right track of starting to wonder why the thing I thought was a relation
> turns out not to be.

Of course, in the above, basic::subs should be ex::subs. I was still
working with an old version of GiNaC.