List of options to be matched {a|b|c|d}
Repeat (one or more) (a)...
Capture group into a symbol name [name(a)]
Matches anything as a wildcard. Used for bare names like x.
Force the match to be at the start [^], or at the end [$].
Match a value with a certain ThingType [:type]
Match that literal value [=value]
Sequence of things in order
abcd