A nice short read for an introduction to equational logic: http://www.cs.cornell.edu/home/gries/Logic/Equational.html http://www.cs.cornell.edu/home/gries/Logic

A nice short read for an introduction to equational logic: http://www.cs.cornell.edu/home/gries/Logic/Equational.html http://www.cs.cornell.edu/home/gries/Logic/Equational.html The Leibniz rule in particular carries a lot of weight.