For the reproduction shake only assert.decreasesBy. See reproduction, line 16 pass; 18, 19 both fail.

AssertionError: 1: expected [Function: fn] to have property 'x'

But i suspecting:

• assert.decreasesButNotBy
• assert.increasesBy
• assert.increasesButNotBy

When use getter function, let me know if i am wrong.
Discovered when made #1085 to aim #1084.