code: def p_if_sentence(p):
      
      cond_ = p[1]
      if len(p) == 6:
          lbl = make_label(p[3], p.lineno(3))
          stat_ = make_block(lbl, p[4])
          endif_ = p[5]
      elif len(p) == 5:
          stat_ = p[3]
          endif_ = p[4]
      else:
          stat_ = make_nop()
          endif_ = p[3]
  
      p[0] = make_sentence('IF', cond_, make_block(stat_, endif_), lineno=p.lineno(2))
docs: statement : if_then_part NEWLINE program_co endif
                    | if_then_part NEWLINE endif
                    | if_then_part NEWLINE statements_co endif
                    | if_then_part NEWLINE co_statements_co endif
                    | if_then_part NEWLINE LABEL statements_co endif