Statement modifiers qualify or restrict a statement.  They let you execute
  a  statement  conditionally or create an implied loop.  A modifier affects
  only  the  statement  immediately  preceding  it.   You  can  modify  only
  executable statements.

  The FOR, UNTIL, and WHILE modifiers create a loop on  a  single  line.   A
  statement  modified  by  FOR  executes until the loop variable reaches its
  upper limit.  A statement modified by UNTIL executes as long  as  cond-exp
  is  false.   A statement modified by WHILE executes as long as cond-exp is
  true.

  The IF and UNLESS modifiers test a conditional  expression.   A  statement
  modified  by  IF  executes  only  when  cond-exp is true.  No THEN or ELSE
  clause can follow  the  IF  modifier.   A  statement  modified  by  UNLESS
  executes only when cond-exp is false.

  Example

  PRINT X IF X <> 0

  Format

      statement modifier cond-exp