If statements can detect if the given value is defined or undefined. Undefined means that the value is empty (e.g., there is nothing provided). Defined means the value is not empty.
Arguments
defined=?(value): where the following lines before EndIf or Else are used if the given (value) is not empty. Otherwise, use any lines between the Else and EndIf.
undefined=?(value): same as above, except the following lines are used if the given (value) is empty.