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Inlining a message call is a refactoring that 'cuts out the middle man'.
To inline a message call
When a message call X is inlined, any calls that were made by the receiver of X will now be made by X's sender and X itself will have been deleted. In effect, a responsibility is moved from receiver to sender.
For example, suppose we inline 'call to target 2' in the following diagram :
The result is that 'call to target3' and 'call to target4' are now be made by target1 :
Note that the inlined call itself is deleted from the diagram. You can always undo this change of course.
If you only want to inline a part of a message call, you'll have to split its activation first and then inline just the part you want.