`wt-tree/union``tree1``tree2`procedureReturns a new tree containing all the associations from both

*tree1*and*tree2*. When both trees have an association for the same key, the returned tree contains the one from*tree1*.The worst-case time required by this operation is proportional to the sum of the sizes of both trees. If the minimum key of one tree is greater than the maximum key of the other tree then the time required is at worst proportional to the logarithm of the size of the larger tree.