In a forest when one tree grows taller the adjacent tree will also strive to grow taller and vice versa. We can define two mutually influencing tree machines M1 and M2 such that each will propagate to the other the solution(s) it has found, and adapt from the other the solution(s) the other has found. The concentrator is used after the eliminator to pick the final solution.

M1:
cycle11: [on non-empty P15 switch to cycle11 with initial problem set P15]
P10 +adap= P11 -enum< P12 >elim- P13 >conc= P14 =prop+ P15

M2:
cycle21: [on non-empty P25 switch to cycle21 with initial problem set P25]
P20 +adap= P21 -enum< P22 >elim- P23 >conc= P24 =prop+ P25