I like that a lot, especially since it draws the two critical elements of variable and assignment together. On a minor note, both := and = are currently the same color: black. That only furthers the ":= is a message" misconception. It might be worthwhile also giving := its own color, given its uniqueness.

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I think you could do nested underlining in the same way that nested blocks and parentheses work. someMessage: and the second and: are underlined in black. otherMessage: and the first and: are underlined in green. That way nesting is clear. This would also be useful for code with nested conditionals, probably the most common occurrence of nested multi-part messages. I also like (c) as a minimally intrusive change that could help novices when they write code. It has the disadvantage that novices couldn't look at a piece of foreign code and get where the multi-part messages were.�

Cheers,

Jeff