|x| from 20 points in [-1, 1], with real and Boolean types
1000 trees, until the RMSE is at most 3.39e-11

Target after 2 generations and 2600 evaluations
RMSE on the 20 training points: 0.00e0
RMSE on the 101 test points:    0.00e0

the expression, 10 nodes of depth 2:
if(less(x, 0.0), mul(-1.0, x), sub(x, 0.0))
