Conditions for Uniqueness of Left Inverse Mapping/Warning

Warning on Conditions for Uniqueness of Left Inverse Mapping

In some expositions of set theory, the case where $S$ is a singleton is not mentioned.

This should be considered a mistake.

Such sources include:

1967: George McCarty: Topology: An Introduction with Application to Topological Groups

1996: H. Jerome Keisler and Joel Robbin: Mathematical Logic and Computability