Expands each element of this Iterable into zero or more elements.
The resulting Iterable runs through the elements returned
by f for each element of this, in iteration order.
The returned Iterable is lazy, and calls f for each element
of this every time it's iterated.
Example:
var pairs = [[1, 2], [3, 4]];
var flattened = pairs.expand((pair) => pair).toList();
print(flattened); // => [1, 2, 3, 4];
var input = [1, 2, 3];
var duplicated = input.expand((i) => [i, i]).toList();
print(duplicated); // => [1, 1, 2, 2, 3, 3]
Source
Iterable<T> expand<T>(Iterable<T> f(E element)) =>
new ExpandIterable<E, T>(this, f);