Union()

[elliotchance]

Unlike Append() and Extend(), Union() always considered the slice empty:

pie.Strings{}.Union(pie.Strings{"baz", "qux"}, pie.Strings{"quux"})
// pie.Strings{"baz", "qux", "quux"}

pie.Strings{"foo", "bar"}.Union(pie.Strings{"baz", "qux"}, pie.Strings{"quux"})
// pie.Strings{"baz", "qux", "quux"}

This syntax is useful when unioning multiple slices without separating the first one from the rest:

// This is easier:
pie.Strings{}.Union(zeroOrMoreSlices...)

// Than handling the nil case:
if len(zeroOrMoreSlices) == 0 {
    return nil;
}

return zeroOrMoreSlices[0].Extend(zeroOrMoreSlices[1:]...)