scm stands for single component M-H

To use, you must also define the following preds, 
scm_gen_each_vid( Model, Id ). 
	On backtracking generate all alteration Ids for Model.
scm_gen_vstructure( A, B, C, D, E ).
scm_gen_vstructure( MOD, Id, VMOD, V, Old )
	part of MOD identified by ID, is replaced with variable V to give VMOD.
	Old, is the replaced part.
	
scm_rel_likelihood/3
	scm_rel_likelihood( NewFamily, OldFamily, RelL )

See bns/src/bns_scm.pl .
