code: public BtrpOperand getImportable(String label, String namespace) {
          if (canImport(label, namespace)) {
              return exported.get(label);
          }
          return null;
      }
docs: Get the exported operand from its label.
  
  @param label     the operand label
  @param namespace the namespace of the script that ask for this operand.
  @return the operand if exists or {@code null}