add methods to query representation of symbolic expressions

[burcin]

Attached patch adds _is_symbol(), _is_constant() and _is_numeric() methods to sage.symbolic.expression.Expression objects.

These methods are just a thin wrapper around the is_a<*>() methods from pynac. They should provide a straightforward interface to query the internal representation of a symbolic expression when .operator() returns None.

Some relevant discussion on sage-devel:

http://groups.google.com/group/sage-devel/msg/6323b473af195bc7

CC: @sagetrac-fmaltey

Component: symbolics

Author: Burcin Erocal

Reviewer: Mike Hansen

Merged: sage-4.3.rc0

Issue created by migration from https://trac.sagemath.org/ticket/7603

[burcin]

Attachment: trac_7603-is_symbol.patch.gz

[mwhansen]

comment:2

Looks good.

[mwhansen]

Merged: sage-4.3.rc0

[mwhansen]

Reviewer: Mike Hansen