code: public <T extends Number> IfNotExistsFunction<NS> ifNotExists(Set<T> defaultValue) {
          return new IfNotExistsFunction<NS>(this, new LiteralOperand(defaultValue));
      }
docs: Returns an <code>IfNotExists</code> object which represents an <a href=
  "http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html"
  >if_not_exists(path, operand)</a> function call where path refers to that
  of the current path operand; used for building expressions.
  
  <pre>
  "if_not_exists (path, operand) – If the item does not contain an attribute
  at the specified path, then if_not_exists evaluates to operand; otherwise,
  it evaluates to path. You can use this function to avoid overwriting an
  attribute already present in the item."
  </pre>
  
  @param defaultValue
  the default value that will be used as the operand to the
  if_not_exists function call.