implicit TRUE and FALSE
The notion of TRUE and FALSE are implicit in many statements, including:

IF , IFZ , CASE , DO WHILE , DO UNTIL , LOOP WHILE , LOOP UNTIL .

The following examples illustrate the nature of TRUE and FALSE and how convenient they can be.

  a = 0       ' a is FALSE because its value is zero
  b = -3      ' b is TRUE because its value is non-zero
  c = 2       ' c is TRUE because its value is non-zero
  a$ = ""     ' a$ is FALSE because it has no contents
  b$ = "0"    ' a$ is TRUE because it has contents ("0")
  c$ = " "    ' a$ is TRUE because it has contents (" ")
  d$ = "\0"   ' a$ is TRUE because it has contents (1 byte)
  DIM a[]     ' a[] is FALSE because it has no contents
  DIM b[0]    ' b[] is TRUE because it has contents
  DIM c[1]    ' c[] is TRUE because it has contents
  DIM d$[7,]  ' d$[] is TRUE because it has contents
'
' note: d$[0,] is FALSE because it has no contents
'
  IF a THEN PRINT a          ' Nothing will print because a is FALSE
  IF b THEN PRINT b          ' "-3" will print because b is TRUE
  IF c THEN PRINT c          ' "2" will print because c is TRUE
  IF a$ THEN PRINT "a$"      ' Nothing will print because a$ is FALSE
  IF b$ THEN PRINT b$        ' "0" will print because b$ is TRUE
  IF c$ THEN PRINT "c$"      ' "c$" will print because c$ is TRUE
  IF d$ THEN PRINT "d$"      ' "d$" will print because d$ is TRUE
  IF a[] THEN PRINT "a[]"    ' Nothing will print because a[] is FALSE
  IF b[] THEN PRINT "b[]"    ' "b[]" will print because b[] is TRUE
  IF c[] THEN PRINT "c[]"    ' "c[]" will print because c[] is TRUE
  IF d$[] THEN PRINT "d$[]"  ' "d$[]" will print because d$[] is TRUE
  IF d$[1] THEN PRINT "YES"  ' Nothing will print because d$[1] is FALSE
'
  DO WHILE b                 ' The loop will execute because b is TRUE
    INC b                    ' b = b + 1
    IF c THEN PRINT "hi"     ' "hi" will print because c is TRUE
  LOOP WHILE a$              ' The loop will end because a$ is FALSE
'
  SELECT CASE ALL TRUE       ' Do all CASEs that are true
    CASE a: PRINT "a"        ' Nothing will print because a is FALSE
    CASE b: PRINT "b"        ' "b" will print because b is TRUE
    CASE a, b: PRINT "a, b"  ' "a, b" will print because b is TRUE
    CASE a, a$: PRINT a, a$  ' Nothing will print because a and a$ are FALSE
    CASE a[]: PRINT "a[]"    ' Nothing will print because a[] is FALSE
    CASE b[]: PRINT "b[]"    ' "b[]" will print because b is TRUE
    CASE d$[]: PRINT "d$[]"  ' "d$[]" will print because d$ is TRUE
    CASE d$[e]: PRINT d$[e]  ' Nothing will print because d$[e] is FALSE
  END SELECT