A forward range of elements.
An input range of elements.
A slice of r1 which contains the characters that both ranges start with, if the first argument is a string; otherwise, the same as the result of takeExactly(r1, n), where n is the number of elements in the common prefix of both ranges.
assert(commonPrefix("hello, world", "hello, there") == "hello, ");
Returns the common prefix of two ranges.