“ … Undoubtedly, the agreement, which was the foundation of the cause of action, expressly refers to a series of transactions between the vendor and defendants 2 to 20 and the common question of fact involved in the suit was whether defendant 1 had the sole title to sell the property to the plaintiff and whether the contract of sale affected the interests of defendants 2 to 21. It is indeed true that in a suit for specific performance the general rule is that a stranger to the contract cannot be sued upon it. Only the parties to the contract are, according to that rule, necessary and sufficient parties. But the general rule is subject to certain modifications (see Halsbury's Laws of England, Edn.2, Vol.31, Para.497, P.417, and the foot-notes thereunder. Se also Fry's Specific Performance of Contracts Edn.6 p.90, para, 205). For instance, strangers are made parties as an exception to the rule in cases of novatio; in cases of an interest arising under a prior contract; and in cases where it is desirable to avoid multiplicity of suits. That is exactly what O.1, R.1, Civil P.C., contemplates. According to Fry (paras 206 and 209) :