Consider the three-level pattern grammar (see "-gr.tryDESTRU"):

RND

<2-1> S --> (= (= X) S (:X)) (: (= X) S (:X))

<2-1> X --> Y

<2-1> X --> Y S Z

<2-1> X --> Z

----------------------------------------

ORD

LEFT S --> lambda

----------------------------------------

LIN

[A line is missing here...]

X X --> abca

Z Y --> abc

Y Z --> cba

Y Y --> cbbc

Z Z --> lambda

This grammar would not produce strings of terminal symbols because the first subgrammar produces strings of X's and Y's mixed with structural symbols "(=", "(:", ")". These symbols should be deleted before subgrammar #3 is applied. Procedure "_destru" takes care of it. This instruction should be placed on top of the third subgrammar:

LIN

_destru

X X --> abca

Z Y --> abc

...

With the "_destru" procedure, terminal strings on the alphabet {a,b,c} are generated, for instance:

cbbcabcabcabcabccbbcabcabcabcabc

abcabcabcabcabcabcabcabc

cbacbbccbbccbacbacbbccbbccba

cbacbacbacba

cbaabccbaabc

cbbccbbccbbccbbccbbccbbc

abcabcabcabcabcabcabcabc