code: public boolean declareImmutable(String label, BtrpOperand t) {
          if (isDeclared(label)) {
              return false;
          }
          level.put(label, -1);
          type.put(label, t);
          return true;
      }
docs: Declare an immutable variable. The variable must not
  has been already declared.
  
  @param label the identifier of the variable
  @param t     the operand associated to the identifier
  @return {@code true} if the variable as been declared. {@code false} otherwise