This is another proof that Vim is perfectly compatible with Vi. The URM macro package was written by Rudolf Koenig ("Rudi") (rudolf@koeniglich.de) for hpux-vi in August 1991. Getting started: type in your shell: vim urm in vim: :so urm.vim in vim: * (to load the registers and boot the URM-machine :-) in vim: g (for 'go') and watch the fun. Per default, 3 and 4 are multiplied. Watch the Program counter, it is visible as a comma moving around. This is a "standard URM" (Universal register machine) interpreter. The URM concept is used in theoretical computer science to aid in theorem proving. Here it proves that vim is a general problem solver (if you bring enough patience). The interpreter begins with register 1 (not 0), without macros and more-lines capability. A dot marks the end of a program. (Bug: there must be a space after the dot.) The registers are the first few lines, beginning with a '>' . The program is the first line after the registers. You should always initialize the registers required by the program. Output register: line 2 Input registers: line 2 to ... Commands: a increment register s decrement register ; execute command and then () execute command while register is nonzero . ("dot blank") halt the machine. Examples: Add register 2 to register 3: (a2;s3)3. Multiply register 2 with register 3: (a4;a5;s2)2; ((a2;s4)4; s3; (a1;a4;s5)5; (a5;s1)1)3. There are more (complicated) examples in the file examples. Note, undo may take a while after a division.