Ts ::= Ts D T (0.2) | T (0.5)
T ::= IM7 (0.1) | I (0.9)
D ::= V7 (1.0)
I ::= I I_int (0.9) | I_int (0.1)
V7 ::= V7 V7_int (0.9) | V7_int (0.1)
I_int ::= c | e | g
V7_int ::= d | f | g | b