##### State Referents #####
+ -> Tests for functionality "Evolve ALPACA CA one generation"
* the name of a defined state, to refer to that state directly
* a chain of _arrows_, to refer to the state of the cell found at
that relative position in the playfield
-Example: a somewhat less simple ALPACA description. Here the states
-have transition rules that cause each cell to take on the state of the
-cell to the "north" (immediately above it.) The effect would be to
-make any form in this cellular automaton "scroll downwards":
+Example: a somewhat less simple ALPACA description. Here the (non-empty)
+states have transition rules that cause each cell to take on the state of one
+of its neighbours; `Up` cells take on the state of the cell to the "north"
+(immediately above it) while `Down` cells take on the state of the cell to
+the "south" (immediately below it.) The effect in this example is for the
An arrow is either `^` (referring to one cell "north" or "above" the
current cell,) `v` (one cell "south" or "below",) `<` (one cell "west"
Example: an ALPACA description of a cellular automaton where `Thing`
elements grow "streaks" to the northwest (diagonally up and to the left.)
| to Thing when v> Thing;
##### Boolean Expressions #####
+ -> Tests for functionality "Parse ALPACA Description"
The boolean expression may be:
* the constant `true` or the constant `false`