Vim gem: built-in calculation

Vim is my go-to editor. Has been for 20 years. Besides being an all-around awesome editor for composing text, it also has some handy built-ins, like calculations:

• In insert mode, ^R= accepts a mathematical expression, the result of which will be inserted in place.
• In normal mode, ^A increments the number at (or to the right of) the cursor by one, while ^X decrements it by one. These accept repeats, so 5^A will add 5 to the number.