• Also, I feel thanks to thinking about combinatorial objects for a while I've gained some kind of "confidence", where I check a special case which I am confident generalizes well.

void editor_state_backspace_char(EditorState& s) {    assert(s.loc.line <= s.contents.size());    if (s.loc.line == s.contents.size()) { return; }    std::string& curline = s.contents[s.loc.line];    assert(s.loc.col <= curline.size());    if (s.loc.col == 0) { return; }    // think about what happens with [s.loc.col=1]. Rest will work.    std::string tafter(curline.begin() + s.loc.col, curline.end());    curline.resize(s.loc.col - 1); // need to remove col[0], so resize to length 0.    curline += tafter;    s.loc.col--;}