--- step1_read_print ----------------------------
import reader, printer

READ(str): return reader.read_str(str)

EVAL(ast,env): return ast

PRINT(exp): return printer.pr_str(exp)

rep(str): return PRINT(EVAL(READ(str),""))

main loop:
  try:      println(rep(readline("user> ")))
  catch e:  println("Error: ", e)