for T1, T2 being Tree
for p being Element of T1 \/ T2 holds
( ( p in T1 & p in T2 implies (T1 \/ T2) | p = (T1 | p) \/ (T2 | p) ) & ( not p in T1 implies (T1 \/ T2) | p = T2 | p ) & ( not p in T2 implies (T1 \/ T2) | p = T1 | p ) )