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A double-shafted arrow between functors.
Also known as: Functor transformation arrow, Two-cell arrow, Doubled-shaft arrow
A doubled arrow shaft marks a natural transformation — a map between functors rather than between objects. The level of the arrow is what the notation encodes: single for a function, double for a map between the things that map.
Denotes a natural transformation between two functors, one level above an ordinary map.
Contexts: Mathematics
Uses the doubled shaft to record that level, which is the only visual difference from an ordinary function arrow.
Contexts: Mathematics
A horizontal arrow whose shaft is drawn as two parallel lines, ending in a single pointed head.
The same doubled arrow means logical implication elsewhere.