CheckMap checks, through reflection, to see if m is a map with string keys.

JoinNonEmpty joins the non-empty strings in xs using sep.

MapKeys gets the keys of a string map m as a slice.

NewSet constructs a set from the elements xs.

PluralQuantity formats a quantity n using the components stem, one, and many as follows: if n is 1, we return '[n] [stem][one]'; else, '[n] [stem][many]'.

SafeMapKeys is the same as calling MapKeys on m, but checks to make sure m is a string map first.

ErrNotMap occurs when a generic string map function receives something that isn't a string map.

Set is a string set, implemented in the usual Go way as a map to empty structs.