程序通过了 但是有这样的错误 求解释

maple304481619

> s := proc (x, y) local i, x0, y0, f, fm; option remember; x0 := 0; y0 := 0; fm := 0; f := proc (k) options operator, arrow; 0 end proc; for i from 0 to x+y do if 0 <= fm and 0 <= x and 0 <= y then x0 := x0+1, fm := fm-y, f := f+ii(k-x0) elif 0 <= fm and 0 <= x and y < 0 then x0 := x0+1, fm := fm-y, f := f+ii(k-x0) elif 0 <= fm and x < 0 and 0 <= y then x0 := x0-1, fm := fm-y, f := f+ii(k-x0) elif 0 <= fm and x < 0 and y < 0 then x0 := x0-1, fm := fm-y, f := f+ii(k-x0) elif fm < 0 and 0 <= x and 0 <= y then y0 := y0+1, fm := fm+x, f := f+ii(k-y0) elif fm < 0 and 0 <= x and y < 0 then y0 := y0-1, fm := fm+x, f := f+ii(k-y0) elif fm < 0 and x < 0 and 0 <= y then y0 := y0+1, fm := fm+x, f := f+ii(k-y0) else y0 := y0-1, fm := fm+x end if end do; return f end proc;
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                            Error, `:=` unexpected
请问哪里有错啊，刚学这个软件，还不太熟。