using primitives
$join []
$join [[1,2], [3,4,5], [6,7]]
$join {{1}, {2}, {3}}
$join {{1,2}, {2,3}, {1,3}}
$join ⟅⟅1⟆, ⟅1,2⟆, ⟅2,3⟆, ⟅1,1,3⟆⟆
$join ⟅⟅1⟆, ⟅1⟆, ⟅1,2⟆, ⟅2,3⟆, ⟅1,2⟆, ⟅1,2⟆, ⟅1,1,3⟆⟆
$join ($join (bag [bagFromCounts {(⟅1⟆, 2)}, bagFromCounts {(⟅1,2⟆, 3)}, bagFromCounts {(⟅2,3⟆, 1)}, bagFromCounts {(⟅1,1,3⟆, 5)}]))