It keeps up with List at List's own game

A result I still find faintly absurd. Because +: is O(1) and the tree is lazy, you can drive FArray as a literal cons-list (build it with ::, take it apart with case h :: t, recursively) and it keeps pace with List, the structure that exists for precisely that.

Terms first, for the Java reader: ::, pronounced "cons", is List's cell, a head plus a tail pointer, so x :: xs prepends in O(1) and case h :: t pattern-matches a list apart into head and tail. +: is the same prepend on FArray.

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The code is character-for-character what you'd write for List. On pure tear-down recursion it's a dead heat; on the cons-build it's about twice as quick.

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