module Map =    
    /// Left join lhs map with rhs using a rhs map key selector
    let leftJoin2 joinOn left right =
      let leftKeyVal = Map.toSeq left
      seq {
        for (key, lv) in leftKeyVal do
          let joinKey = joinOn (key, lv)
          let rightValueOpt = Map.tryFind joinKey right
          yield key, lv, rightValueOpt
        }
      
    /// Inner join lhs map with 2 rhs maps with 'k key selection on lhs map
    let innerJoin3 joinOn left (right1: Map<'k,_>) (right2: Map<'k,_>) =
      let leftKeyVal = Map.toSeq left
      seq {
        for (key, lv) in leftKeyVal do
          let joinKey = joinOn (key, lv)
          let r1Value = Map.tryFind joinKey right1
          let r2Value = Map.tryFind joinKey right2
          match (r1Value, r2Value) with
          | Some r1, Some r2 -> yield key, lv, r1, r2
          | _ -> ()
        }