s_expression = atomic_symbol
              / "(" s_expression "." s_expression ")"
              / list 

list = "(" s_expression < s_expression > ")"

atomic_symbol = letter atom_part

atom_part = empty / letter atom_part / number atom_part

letter = "a" / "b" / " ..." / "z"

number = "1" / "2" / " ..." / "9"

empty = " "
