ALLAN SUGAR (PRODUCTS) LIMITED v. GHANA EXPORT COMPANY LIMITED Deeds and documents—Construction—Parol evidence—Admissibility—Parties bound by written agreement—Circumstances where parol evidence acceptable as aid to construction—AS Ltd. and GE Ltd. obtaining lease of separate parts of irrigated land from NIB—GE Ltd. granted exclusive user of irrigation facilities—Recommendation by government that irrigation system be put to commercial use rejected by NIB—Government not a contracting party—Whether NIB or GE Ltd. bound by suggestion of government—Whether antecedent or subsequent negotiation admissible to vary agreement of parties.