private String compare(String leftHandSide, String rightHandSide) {
        if (leftHandSide.equals(INTEGER)) {
            if (rightHandSide.equals(INTEGER)) {
                return INTEGER;
            } else if (rightHandSide.equals(REAL)){
                return REAL;
            } else {
                SemanticWarningList.addWarning(SemanticWarning.makeNewWarning(
                        "Unknown type for variable '" + rightHandSide + "'. Undeclared variable."));
            }
        } else if (leftHandSide.equals(REAL)) {
            if (rightHandSide.equals(INTEGER) || rightHandSide.equals(REAL)) {
                return REAL;
            } else  {
                SemanticWarningList.addWarning(SemanticWarning.makeNewWarning(
                        "Unknown type for variable '" + rightHandSide + "'. Undeclared variable."));                
            }
        } else {
            SemanticWarningList.addWarning(SemanticWarning.makeNewWarning(
                    "Unknown type for variable '" + leftHandSide + "'. Undeclared variable."));
        }
        return EMPTY;
    }