How find works?

[Paul Royik]

Can somebody explain how find works.

For example,

a = sympy.Wild('a', exclude=[x])

b = sympy.Wild('b', exclude=[x])

c = sympy.Wild('c', exclude=[x])

pattern = sympy.sqrt(a*x**2+b*x+c)

1/((x^2+a^2)+sqrt(a^2-x^2)).find(pattern) # -> set([sqrt(a^2-x^2)])

1/((x^2+a^2)*sqrt(a^2-x^2)).find(pattern) # -> set([])

How is that possible?