Paste: Stack effect error

! x and y are numbers, boards are 2d arrays

SYMBOL: _

: marker-at ( x y board -- marker ) nth nth ;                                                                                                                                     
: occupied? ( x y board -- ? ) marker-at _ eq? not ;                                                                                                                              
:: in-bounds? ( x y board -- ? ) x y [ 0 >= ] bi@ and                                                                                                                             
    x y [ board length < ] bi@ and and ;                                                                                                                                          
: available? ( x y board -- ? ) 3dup in-bounds? [ occupied? not ] when ; 

! The word available? cannot be executed because it failed to compile 
!
! The input quotation to “when” doesn't match its expected effect
! Input             Expected       Got
! [ occupied? not ] ( ..a -- ..a ) ( x x x -- x )

Annotation: false branch needs to conform to stack declaration

! Your true and false branches for the available? word 
! are not balanced. The true branch conforms to the 
! stack effect, but the false branch leaves the x y 
! and board elements which violates the stack 
! effect declaration. 

! The following definition balances both branches: 

: available? ( x y board -- ? ) 3dup in-bounds? [ occupied? not ] [ 3drop f ] if ; 

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