Here's how monospace ligatures work in real life: the width of a glyph is strictly proportional to how many ASCII bytes it represents. A 1-byte glyph like `m` gets 1 cell. A 2-byte glyph like `->` gets 2 cells. A 3-byte glyph like `===` gets 3 cells.

1 ASCII byte, 1 cell. That way ligatures don't change the width of the original text. If a line used to require 42 columns, it still requires 42 columns. That's why we call them "monospace": each ASCII byte requires the exact same width, even when it's part of a ligature.

> a compacting ligature for "www" —

Is obviously out of scope. We're discussing monospace ligatures here, not compacting ones.