comint-get-old-input is a variable defined in `comint.el'.
Its value is

Function that returns old text in Comint mode.
This function is called when return is typed while the point is in old
text. It returns the text to be submitted as process input. The
default is `comint-get-old-input-default', which either grabs the
current input field or grabs the current line and strips off leading
text matching `comint-prompt-regexp', depending on the value of