code: public void load(String formula, Engine engine) {
          this.root = parse(formula);
          this.formula = formula;
          this.engine = engine;
      }
docs: Loads the given formula expressed in infix notation, and sets the engine
  holding the variables to which the formula refers.
  
  @param formula is the right-hand side of a mathematical equation expressed
  in infix notation
  @param engine is the engine to which the formula can refer
  @throws RuntimeException if the formula has syntax errors