Programmed grammars in BP2 use flags taking integer values. Suppose we need to generate random variations of length 6 containing 'a' and 'b', yet with exactly four occurrences of 'a' and two of 'b'. We may write (see "-gr.tryflags"):

RND

GRAM#1[1] S --> X X X X X X /flag1 = 4/ /flag2 = 2/

GRAM#1[2] /flag1 -1/ X --> a

GRAM#1[3] /flag2 -1/ X --> b

The first rule generates "X X X X X X" and six flags. More precisely, "flag1" is set to integer value 4 whereas "flag2" is set to 2.

These are used as conditions for the application of rules [2] and [3]: a rule may be applied only if the values of all its condition flags are strictly positive . In addition, each time rule [2] (or [3]) is applied, "flag1" (or "flag2") is decremented by one unit.

Productions look like:

a a a a b b, b a a a b a, b a a a b a, a a b a b a, a b a b a a, b a b a a a, a b a a a b...

Another example: generate a string of length 10 on alphabet {a,b,c} in which 'b' and 'c' have equal numbers of occurrences (±1). The following grammar
("-gr.tryflags2") will do it:

[1] S --> X X X X X X X X X X /make_b = 1/

[2] /make_b -1/ X --> b /make_c +1/

[3] /make_c -1/ X --> c /make_b +1/

[4] X --> a

This grammar produces:

c a b b b c c a a a, a b a a b a a c c b, b a b c a b b c c a, c a b a a a b a a c...

We recommend reading about production procedures (reference manual §8.1) that solve equivalent problems.