As an equitable doctrine, the court has discretion.
One might suppose that since they now come into a court with an equitable jurisdiction, the equitable doctrine must be applied.
As an equitable doctrine, the court has the discretion to vitiate such a contract.
Breach of confidence in English law is an equitable doctrine which allows a person to claim a remedy where their confidence has been breached.
Undue influence is an equitable doctrine that involves one person taking advantage of a position of power over another.
However, there are two affirmative defenses that have been accepted by some courts: state sovereign immunity and the equitable doctrine of laches.
They were held to be at a "special disadvantage" as an equitable doctrine in Equity (law).
This does not call for the introduction of any equitable doctrine.
That equitable doctrine, however, could hardly apply in the present case because the variation here might be said to have been made without consideration.
This old rule is full of exceptions, particularly where people wished to vary their agreements, through case law and the equitable doctrine of promissory estoppel.