pchiusano/fpinscala — pull request #137 by mirzov (2013-07-03 21:40:54+00:00)
Title: Correction for answerkey/monoids/5.answer.scala

The original implementation does not satisfy the laws for zero:
op("", " a") = "a" != " a".

The proposed implementation uses " " for zero and performs explicit checks whether operands are zero.