    # KEHOME/doc/father.html
    # Aug/5/2003

    #===============================#
    # user-defined binary relations #
    # example: father               #
    #===============================#

    1. father as an attribute
    Many people would simply represent "father" as an attribute.

    	Dick has father = Mac;

    2. father as an n-ary relation
    However, "father" is really a relation between entities,
    not an attribute of a single entity.

    	father is relation with
    	    format = [person:1, person:2];
    	begin relation father;
    	Dick, Mac;
    	end relation father;

    3. father as a binary relation
    (This syntax was first introduced in MKR version 6.0.)
    A simpler description is obtained using a binary relation.

    	Dick rel father = Mac;

    To minimize the differences between user-defined binary relations
    and built-in MKR binary relations, this alternate syntax is allowed
    (but only recommended for important MKR "extensions").

    	Dick father Mac;

    Note that the "father" of 3 is not the same as the "father" of 2.
    The "father" of 2 is automatically made available as "father_rel".

    	father_rel is relation with
    	    automatic = "none",
    	    format = [existent:1, existent:2],
    	    formattype = verb,
    	    formatverb = father;
    	begin relation father_rel;
    	Dick, Mac;
    	end relation father_rel;