Algorithm 3. Function for randomization step of the GSCM.

1:	function pickRandomTreePair (trees $\mathcal{S}\subseteq \left\{T_1,T_2,\dots ,T_k\right\}$)
2:	Randomly pick a pair of trees $\left\{T_i,T_j\right\}\subseteq \mathcal{S}$ with probability
	$P\left(T_i,T_j\right)=\frac{score\left(T_i,T_j\right)}{{\sum }_{T_a,T_b\in \mathcal{S},a\ne b}score\left(T_a,T_b\right)},i\ne j$
3:	return Ti, Tj
4:	end function