ds1,ds2:Collection(dec()), da1,da2:Collection(SimpleType), de:sig()
, t:Term. ds1 
ds2 
da1 da2 term_types(ds1;da1;de;t) term_types(ds2;da2;de;t)c1,c2:Collection(T). c1 = c2 c1 c2x:Label, a1,a2:Collection(dec()).
a1 a2 dec_lookup(a1;x) dec_lookup(a2;x)a1,a2,b1,b2:Collection(SimpleType).
a1 a2 b1 b2 st_app(a1;b1) st_app(a2;b2)