code: public final boolean declare(String label, BtrpOperand t) {
          if (isDeclared(label) && level.get(label) < 0) { //Disallow immutable value
              return false;
          }
          if (!isDeclared(label)) {
              level.put(label, currentLevel);
          }
          type.put(label, t);
          return true;
      }
docs: Declare a new variable.
  The variable is inserted into the current script.
  
  @param label the label of the variable
  @param t     the content of the variable
  @return {@code true} if the declaration succeeds, {@code false} otherwise