Editing Monotonicity of entailment (section)

==Weakening rule==

Monotonicity may be stated formally as a rule called '''weakening''', or sometimes '''thinning'''. A system is monotonic if and only if the rule is [[admissible_rule| admissible]]. 
The weakening rule may be expressed as a natural deduction sequent: 
:<math>\frac{\Gamma \vdash C}{\Gamma, A \vdash C } </math>
This can be read as saying that if, on the basis of a set of assumptions <math>\Gamma</math>, one can prove C, then by adding an assumption A, one can still prove C.