FLIGHT_NUM
[
VLIB_FLIGHT_NUMBER:z	{<choice $z>}
]

VLIB_FLIGHT_NUMBER (
  ?( flight ?number )
  [
  
    ; one digit with leading zeros
    ( [ double triple ( ?zero zero ) ( ?oh oh ) ] [ zero oh ] 
      VSUB_DIGIT_ONE_THRU_NINE_STRING:n
    ) {return($n)}

    ; two digits with leading zeros
    ( ?[ double zero oh ] [ zero oh ] 
      [ ( VSUB_DIGIT_ONE_THRU_NINE_STRING:t VSUB_DIGIT_STRING:u 
        ) {return(strcat($t $u))}
         VSUB_FLIGHT_NUMBER_10_THRU_99:n {return($n)}
      ]
    ) 

    ; three digits with leading zero
    ( [ zero oh ] VSUB_DIGIT_ONE_THRU_NINE_STRING:h hundred 
    ) {return(strcat($h "00"))}
    ( [ zero oh ] VSUB_DIGIT_ONE_THRU_NINE_STRING:h 
      VSUB_DIGIT_STRING:t VSUB_DIGIT_STRING:u 
    ) {return(strcat($h strcat($t $u)))}

    ; one digit without leading zeros
    VSUB_DIGIT_STRING:n {return($n)}

    ; two digits without leading zeros
    VSUB_FLIGHT_NUMBER_10_THRU_99:n {return($n)}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:t VSUB_DIGIT_STRING:u
    ) {return(strcat($t $u))}

    ; three digits without leading zero
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:h hundred ) {return(strcat($h "00"))}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:h ?( hundred ?and )
      VSUB_FLIGHT_NUMBER_10_THRU_99:u 
    ) {return(strcat($h $u))}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:h [ ( hundred ?and ) zero oh ]
      VSUB_DIGIT_ONE_THRU_NINE_STRING:u 
    ) {return(strcat($h strcat("0" $u)))}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:h VSUB_DIGIT_STRING:t VSUB_DIGIT_STRING:u
    ) {return(strcat($h strcat($t $u)))}

    ; four digits
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:k thousand ) {return(strcat($k "000"))}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:k thousand ?and 
      VSUB_FLIGHT_NUMBER_10_THRU_99:u 
    ) {return(strcat(strcat($k "0") $u))}
    ( VSUB_FLIGHT_NUMBER_10_THRU_99:a VSUB_FLIGHT_NUMBER_THRU_99_WITH_OH:b 
    ) {return(strcat($a $b))}
    ( VSUB_TEEN_STRING:a hundred ) {return(strcat($a "00"))}
    ( VSUB_DECADE_STRING:k VSUB_DIGIT_ONE_THRU_NINE_STRING:h hundred 
    ) {return(strcat(strcat($k $h) "00"))}
    ( VSUB_TEEN_STRING:a hundred ?and VSUB_FLIGHT_NUMBER_1_THRU_99:b
    ) {return(strcat($a $b))}
    ( VSUB_DECADE_STRING:k VSUB_DIGIT_ONE_THRU_NINE_STRING:h hundred ?and
      VSUB_FLIGHT_NUMBER_1_THRU_99:u
    ) {return(strcat(strcat($k $h) $u))}
    ( VSUB_DIGIT_ONE_THRU_NINE_STRING:k VSUB_DIGIT_STRING:h VSUB_DIGIT_STRING:t
      VSUB_DIGIT_STRING:u
    ) {return(strcat(strcat($k $h) strcat($t $u)))}
  ]
)

VSUB_FLIGHT_NUMBER_10_THRU_99 [
  ten {return("10")}
  VSUB_TEEN_STRING:t {return($t)}
  VSUB_DECADE_STRING:t {return(strcat($t "0"))}
  ( VSUB_DECADE_STRING:t VSUB_DIGIT_ONE_THRU_NINE_STRING:u
  ) {return(strcat($t $u))}
]

VSUB_FLIGHT_NUMBER_1_THRU_99 [
  VSUB_FLIGHT_NUMBER_10_THRU_99:n {return($n)}
  VSUB_DIGIT_ONE_THRU_NINE_STRING:n {return(strcat("0" $n))}
]

VSUB_FLIGHT_NUMBER_THRU_99_WITH_OH [
  VSUB_FLIGHT_NUMBER_10_THRU_99:n {return($n)}
  ( oh VSUB_DIGIT_ONE_THRU_NINE:n ) {return(strcat("0" $n))}
]


VSUB_DIGIT_TWO_THRU_NINE [
  two {return(2)}
  three {return(3)}
  four {return(4)}
  five {return(5)}
  six {return(6)}
  seven {return(7)}
  eight {return(8)}
  nine {return(9)}
]

VSUB_DIGIT_ONE_THRU_NINE [
  one {return(1)}
  VSUB_DIGIT_TWO_THRU_NINE:n {return($n)}
]

VSUB_DIGIT [
  [ zero oh ] {return(0)}
  VSUB_DIGIT_ONE_THRU_NINE:n {return($n)}
]

; Single-digit grammars that return string values

VSUB_DIGIT_TWO_THRU_NINE_STRING [
  two {return("2")}
  three {return("3")}
  four {return("4")}
  five {return("5")}
  six {return("6")}
  seven {return("7")}
  eight {return("8")}
  nine {return("9")}
]

VSUB_DIGIT_ONE_THRU_NINE_STRING [
  one {return("1")}
  VSUB_DIGIT_TWO_THRU_NINE_STRING:n {return($n)}
]

VSUB_DIGIT_STRING [
  [ zero oh ] {return("0")}
  VSUB_DIGIT_ONE_THRU_NINE_STRING:n {return($n)}
]

VSUB_TEEN_STRING [
  eleven {return("11")}
  twelve {return("12")}
  thirteen {return("13")}
  fourteen {return("14")}
  fifteen {return("15")}
  sixteen {return("16")}
  seventeen {return("17")}
  eighteen {return("18")}
  nineteen {return("19")}
]

VSUB_DECADE_STRING [
  twenty {return("2")}
  thirty {return("3")}
  forty {return("4")}
  fifty {return("5")}
  sixty {return("6")}
  seventy {return("7")}
  eighty {return("8")}
  ninety {return("9")}
]

VSUB_TEEN [
  eleven {return(11)}
  twelve {return(12)}
  thirteen {return(13)}
  fourteen {return(14)}
  fifteen {return(15)}
  sixteen {return(16)}
  seventeen {return(17)}
  eighteen {return(18)}
  nineteen {return(19)}
]

VSUB_DECADE [
  twenty {return(20)}
  thirty {return(30)}
  forty {return(40)}
  fifty {return(50)}
  sixty {return(60)}
  seventy {return(70)}
  eighty {return(80)}
  ninety {return(90)}
]