It is possible to define a single unique ``compose key'' (<Ctrl>T is used by the Language Supplement) which is used to indicate that the following keystrokes are to be composed by the KBD module into a single character which does not have a key of its own.
For example, if a user requires a u-umlaut, (ü) on the input, they
would use the following sequence of keystrokes:
If the user enters an invalid sequence, such as
no processing is done, and these three characters are simply placed on the input.