code: public function setPrepInstruction($value)
      {
          if (!$this->_isNumericArray($value)) {
              $value = array($value);
          }
          $this->_fields['PrepInstruction']['FieldValue'] = $value;
          return $this;
      }
docs: Set the value of the PrepInstruction property.
  
  @param array $value prepInstruction
  @return $this This instance