auto-composition-function

auto-composition-function is a variable defined in `C source code'.
Its value is

auto-compose-chars

• Automatically becomes buffer-local when set.
• This variable may be risky if used as a file-local variable.

Documentation:
Function to call to compose characters automatically.
This function is called from the display routine with four arguments:
FROM, TO, WINDOW, and STRING.

If STRING is nil, the function must compose characters in the region
between FROM and TO in the current buffer.

Otherwise, STRING is a string, and FROM and TO are indices into the
string. In this case, the function must compose characters in the
string.