x1 is to x2 as x3 is to x4 as x5 is to x6 ...
Takes an even number of arguments (is 2n-ary). Also means: x1 is an x2 type of x3. More widely known as a kenning; x1 : x2 :: x3 : x4 :: x5 : x6... One can use sequences (ce'o) as x2, x4, x6 if more than 2 things form a row.