Atom

@x: Is the argument an atom? This returns either 0 or 1.

  @1
1
  @4 5 6
0
  @`s
1
  @`s`t`u
0
  @ ()
0
  @,1
0
  @ (7;8;`q)
0
  @_n
1
  @{x}
1
  @.((`a;1);(`b;2))
1