 
Summary: 1. A class of good context free grammars.
Suppose
G = (T, N, s, P)
is a context free grammar with the following properties:
(i) there are sets Y , E, O and L such that the family
{Y, E, O, L, N}
is disjointed,
T Y E O L.
(ii) if y Y there is at most one production (r, s) such that y rng s which,
if it exists, has the form
r := y
(iii) for each e E there is at most one production (r, s) such that
e rng s
which, if it exists, is such that
N rng s = , Y rng s = , and s0 = e.
(iv) for each o O there is at most one production (r, s) such that
o rng s
which, if it exists, is such that
N rng s = , Y rng s = , s0 L;
(v) there are no other productions besides those which appear above.
