code: function interval( value, lower, upper ) {
  	if ( !isNumber( value ) || !isNumber( lower ) || !isNumber( upper ) ) {
  		return false;
  	}
  	return ( lower <= value && value <= upper );
  }
docs: INTERVAL // 
  FUNCTION: interval( value, lower, upper )
  Validates if a value resides on a specified interval (inclusive).
  
  @param {Number} value - value to be validated
  @param {Number} lower - interval lower bound (inclusive)
  @param {Number} upper - interval upper bound (inclusive)
  @returns {Boolean} boolean indicating whether value is within a specified interval