'Returns the domain of this composite morphism.
The domain of the composite morphism is the domain of its
first component.
Examples
>>> from sympy.categories import Object, NamedMorphism
>>> A = Object("A")
>>> B = Object("B")
>>> C = Object("C")
>>> f = NamedMorphism(A, B, "f")
>>> g = NamedMorphism(B, C, "g")
>>> (g * f).domain
Object("A")'

@property
def domain(self):

    return self.components[0].domain
