Source : Free On-Line Dictionary of Computing

HEQS
     
        E. Derman.  Constraint language for financial modelling.  Uses
        an extension of the equation solver in IDEAL.  "A Simple
        Equation Solver and Its Application to Financial Modeling",
        E. Derman et al, Soft Prac & Exp 14(12):1169-1181 (Dec 1984).