function twoFens(fen_a, fen_b){
  var i, j, temp, obj_a, obj_b, from_squares, to_squares, coord, is_long_castle, king_rank;

  obj_a=Ic.fenGet(fen_a, "squares activeColor");
  obj_b=Ic.fenGet(fen_b, "squares activeColor");

  if(!obj_a || !obj_b || obj_a.activeColor===obj_b.activeColor){
    return "";
  }

  from_squares=[];
  to_squares=[];

  for(i=0; i<8; i++){
    for(j=0; j<8; j++){
      coord=Ic.toBos([i, j]);

      if(obj_a.squares[coord].val!==obj_b.squares[coord].val){
        if(obj_b.squares[coord].val===0){
          //this excludes en passant capture
          //a.val can't be 0 here, no problem with inverted logic >0 being <=0
          if((obj_a.activeColor==="w")===(obj_a.squares[coord].val>0)){
            from_squares.push(coord);
          }
        }else{
          to_squares.push(coord);
        }
      }
    }
  }

  //4 changes (len=2 + len=2) mean king castled
  //3 changes would mean en passant capture (but we prevented this)
  //1 change (len=1 + len=1) is normal case of moving to empty or normal capture
  if(from_squares.length===2 && to_squares.length===2){
    //you can infer which castling happened in 4 ways:
    //using from_squares and a-file changed = is long castle (used below)
    //using from_squares and h-file changed = is short castle
    //using to_squares and d-file changed = is long castle
    //using to_squares and f-file changed = is short castle
    is_long_castle=(from_squares.join("").indexOf("a")!==-1);
    king_rank=(obj_a.activeColor==="w" ? 1 : 8);

    to_squares=[(is_long_castle ? "c" : "g")+king_rank];
    from_squares=["e"+(obj_a.activeColor==="w" ? 1 : 8)];
  }

  if(from_squares.length!==1 || to_squares.length!==1){
    return "";
  }

  temp=Ic.fenApply(fen_a, "playMove", [[from_squares[0], to_squares[0]]]);

  return (temp ? temp.san : "");
}