Why is this structure declaration allowed in a built-in Function Module but not in a new one?

I'm working on a Function Module to assist with dealing with included text with logic embedded. While looking into the way SAP handles SAPScript files and parses the logic I found a structure that is declared as so:

DATA BEGIN OF events OCCURS 100.
      INCLUDE STRUCTURE ITCCA.
DATA: command LIKE BOOLEAN,
      template LIKE BOOLEAN,
      mask LIKE BOOLEAN,
     END OF events.

This obviously works, as I can trace through it while it is running a print program. So I thought I would try a similar structure in my own code but even when I copied the code 1 for 1 like above I get an error during activation. The error is

"BOOLEAN" must be a flat structure. Internal tables, references, 
strings and structures are forbidden as components.

Can someone explain to me why this structure is valid in one program and not mine?

Solution

To explain the actual effect: LIKE usually refers to a data object (an actual variable) on the right-hand side, not a data type. As you rightly discovered, once you provide a data object named BOOLEAN, that is used to construct the type. If a data object of that name is not present and you're not within a class or an interface, an obsolete variant of the LIKE statement will be triggered that also takes data types into account, but only allows for certain elements on the right-hand side - namely only flat structured objects or their components. LIKE DATATYPE-BOOLEAN should have worked. As usual, the error message is somewhat less than helpful.