ome are not known.
    note: see below for relations between classes.
o  lambda calculus is used for defining constraints. 
    In general these indicate how a member of one class 
    must stand in respect to a member of another class.
    note: relations between classes (e.g. subclass-of) are
    only allowed when the nature of the constraint they
    define is defined by a lambda expression.

Of course I do not claim other choices for other reasons could not be made.

Actually, everything else flows from