A, B, and C are birds such that C composes A with B.
Show that, if C is agreeable, so is A.
Assume an arbitrary bird D. We don't know anything about it, but we want to show that A and D agree on some bird.
Given D, we can assume another arbitrary bird that composes D with B:
const composed = compose(D, B);Because C is agreeable, it and composed agree on at least one bird; call it x:
C(x) === composed(x);When we unpack composed, we get:
C(x) === D(B(x));And when we unpack C (recall, also composed), we get:
A(B(x)) === D(B(x));That is, A and D agree on the bird B(x). Since D is arbitrary, this holds no matter its value; thus, A is agreeable.