In some sections, we may write simply instead of L , L ∅ , NL or NL ∅ .
for X and Y ∈ T (P,≤) , X ≤ Y iif this relation is deductible in the following system where p, q ∈ P , n, k ∈ Z and X, Y, Z ∈ T (P,≤) : 1 This implies that the class has infinite elasticity.