Typewriter Carriage

Replay keystrokes on a manual typewriter and report what ended up on the paper and how many times the margin bell rang.

The sheet has width columns numbered 0 to width - 1, and the carriage starts at column 0 of line 0. The carriage position p can be anywhere from 0 to width; at p = width the carriage is locked at the right margin. Process keys one character at a time:

• A letter, digit, - or _ is typed in column p of the current line, then the carriage moves to p + 1. If the cell was empty, it now holds that character. If it already holds the same character, nothing changes; if it holds a different one, the two overstrike and the cell becomes #. While the carriage is locked, typing does nothing.
• A space moves the carriage to p + 1 without marking the paper (it never changes a cell). While locked, it does nothing.
• < (backspace) moves the carriage one column left, never below 0. It does not erase anything.
• A tab character moves the carriage to the smallest tab stop in tabs that is greater than p. If there is none, it moves to width and locks. While locked, it does nothing.
• \r (carriage return) moves the carriage back to column 0 on the same line.
• \n (line feed) moves the paper up one line, keeping the carriage column.

The bell rings each time a typed character or space (not a tab) moves the carriage from column bell - 1 to column bell. Backspacing and passing that point again rings it again. Typing or spacing that is ignored because the carriage is locked never rings.

Input: width, bell, tabs and keys.

Output: [lines, bells]. lines is the page from line 0 down to the last line that has a mark on it, each as a string of the line's cells with trailing spaces removed (lines in between with no marks are empty strings). If nothing was ever marked, lines is []. bells is the number of rings.