docs: public void setVariable(String variableName, String variableValue, boolean isOptional)
              throws VariableNotDefinedException {
          int varNo = mtp.lookupVariableName(variableName);
          if (varNo == -1) {
              if (isOptional) {
                  return;
              }
              throw new VariableNotDefinedException(variableName);
          }
          varValuesTab[varNo] = variableValue;
      }