  In search of a meaningful 2-dimensional analog to mono- tonicity, we
introduce two new definitions and give examples of and dis- cuss the
relationship between these definitions and others that we found in the
literature. Note: After we published the article in Abstract and Applied
Analysis and after we searched multiple times for previous work, we discovered
that Clarke at al. had introduced the definition of cone monotonicity and given
a characterization. See the addendum at the end of this paper for full
reference information.
