Comment #25 on issue 1337 by mattpap: find all instances in an expression
http://code.google.com/p/sympy/issues/detail?id=1337
subs() and replace() have different semantics. Both get two arguments, but
1. subs() gets a Basic instance, tries to find all its occurrences in an
expression and replace them exactly with another Basic instance.
2. replace() gets some kind of pattern or query and replaces all found
instances with an explicit instance or an instances generated by a function
(possibly also by Lambda).
Example (with find()):
In [1]: a = Wild('a')
In [2]: (x*sin(2*x)).subs(sin(a), cos(2*x))
Out[2]: x⋅sin(2⋅x)
In [3]: (x*sin(2*x)).find(sin(a))
Out[3]: set(sin(2⋅x))
([3] would read (x*sin(2*x)).replace(sin(a), lambda expr: cos(expr.args)),
or it could be even more intelligent, e.g. e.replace(sin(a), cos) with
sub-expression matched by 'a' put into cos(...)).
If we can somehow distinguish expressions with are patterns from real
patterns in subs() then we can think about putting replace()'s
functionality in subs(). However, even in this case, I think that two
functions will be beneficial, especially when we take into consideration
new proposed features to subs(), like exact=True.
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