Crossover with Variable-Length Bitstrings

Given

$a_1$ $a_2$ $c$ $a_1$ $a_2$ $c$

10 01 1 11 10 0

and

$a_1$ $a_2$ $c$ $a_1$ $a_2$ $c$

01 11 0 10 01 0

and the crossover points for the first parent are the points following bit positions 1 and 8

$a_1$ $a_2$ $c$ $a_1$ $a_2$ $c$

1[0 01 1 11 1]0 0

then the allowed pairs of crossover points are \[\langle 1,3 \rangle, \langle 1,8 \rangle, \langle 6,8 \rangle \]
In most GA implementations we have to come up with similar tricks to ensure that offsprings are viable.