completion-base-positioncompletion-base-position is a variable defined in `
simple.el'. Its value is
nilDocumentation: Position of the base of the text corresponding to the shown completions. This variable is used in the *Completions* buffers. Its value is a list of the form (START END) where START is the place where the completion should be inserted and END (if non-nil) is the end of the text to replace. If END is nil, point is used instead.