// kmr_sb1  sandbox

x1 = [ 123,  "abc", #abc, 'abc', `abc ]
x2 =   [`plus, [`lit, 2], [`lit, 5]]
x3 = [ 123 ]

  ometa M1 {
    list0 = [ anything:ins1 anything* ] -> [ins1],
 //   list0 = [ anything* ], 
    list1 = [ 123 ],
    list2 = []
  }

  M1.match(x1, "list0" )
  M1.match([], "list2" )

ometa F1 {
  flatten = [ inside:xs ] -> xs,
  inside  = [ inside:xs ] inside:ys -> ( xs.concat(ys))
|      anything:x  inside:ys        -> ([x].concat(ys))
|      empty                        -> ([])
}

  F1.match([`plus, [`lit, 2], [`lit, 5]], "flatten")
 

 ometa F2 {
  flatten = [ inside:xs ] -> xs,
  inside  = []
|     [ inside:xs ] inside:ys -> ( xs.concat(ys))
|      anything:x   inside:ys -> ([x].concat(ys))
}

  F2.match([], "flatten")  
  F2.match([[]], "flatten") /* [] */ []
  F2.match([[1]], "flatten")
  F2.match([1,2], "flatten")
  F2.match([[1],2], "flatten") 

[1].concat(2)
[1].concat([2])
1/* [] */ 
2+1