sympy/sympy — pull request #1299 by Krastanov (2012-05-15 23:16:13+00:00)
Title: Make Expr recursively callable.

Addresses issue 2006. If an expression is called on an argument it traverses
the expression tree and if an atom is a callable it is called on the argument.
If it is not a callable it is left unchanges:

Examples:

```
>>> (1+x)(y)
1+x

>>> (1+f)(y)
1+f(y)
```