Class Step<FROM, TO, LOST>Abstract

a single step consists a forward transformation from a type to a different type, and a backward transformation that does the reverse.
if information is lost during the forward transformation, it should be stored in the lost member, and then rejoined when the backward transformation is carried out. this will ensure full invertibility of the data.
if no information is lost during the forward transformation, it would be a good idea to use the PureStep subclass instead.