A line editor with undo

The problem

Build a small text editor that keeps the lines of a document in a doubly linked list, and can undo every change.

The lines are kept in a doubly linked list, with a cursor on the current line
Commands insert a line after the cursor, delete the current line, and move the cursor up or down
Every change is recorded on a stack, and an undo command reverses the most recent one
Undo with no history gives a clear message
A print command shows the document with the current line marked
All memory is freed when the program ends

Define the line node, and write functions that insert after the cursor and delete the current line.
Write the print command, and test inserting and deleting at the first, a middle and the last line.
Define an undo record that says what was done and holds what is needed to reverse it.
Push a record on every insert and every delete.
Write undo: pop a record and apply the opposite change, putting the cursor back where it was.
Free the list and the undo stack at the end.
Test a long run of changes followed by the same number of undos, which must give back the empty document.

A working line editor in which any sequence of edits can be undone step by step, with no memory errors.