[m-users.] [Beginner] How to make a function using list pattern matching deterministic

[Razetime]

Currently I have these two predicates in my program that I think I can
make deterministic.

:- pred entry(list(string)::in, list(int)::out) is det.
entry(Dirs, Out) :-
  map((pred(X::in, [L,R]::out) is det :-
         split(X, 1, L, R)),
       Dirs, Spl),
  foldl2((pred([T, B]::in, D::in, Ds::out, P::in, Ps::out) is det :-
            rot(T, D, Ds),
            BI = string.det_to_int(B),
            map((pred(X::in, Y::out) is det :- Y = X*BI), Ds, Dso),
            map_corresponding((pred(X::in, Y::in, Z::out) is det :- Z
= X+Y), P, Dso, Ps)
        ), Spl, [0, 1], _, [0, 0], Out).

:- pred rot(string::in, list(int)::in, list(int)::out) is det.
rot("L", [X, Y], [A, B]) :-
  A = -Y,
  B =  X.
rot("R", [X, Y], [A, B]) :-
  A =  Y,
  B = -X.

I'm having trouble with both of these since they're giving me problems
with determinism inference:

a01.m:017: In `entry'(in, out):
a01.m:017:   Determinism error in lambda expression.
a01.m:017:   Declared `det', inferred `semidet''.
a01.m:017:   In argument 1 of clause head:
a01.m:017:   in argument 2 of functor `[|]/2':
a01.m:017:   in argument 2 of functor `[|]/2':
a01.m:017:   unification with `list.[]' can fail.
a01.m:017:   In argument 1 of clause head:
a01.m:017:   in argument 2 of functor `[|]/2':
a01.m:017:   unification with `list.[B | V_35]' can fail.
a01.m:017:   In argument 1 of clause head:
a01.m:017:   unification of `LambdaHeadVar__1' and `list.[T | V_34]' can fail.
a01.m:024: In `rot'(in, in, out):
a01.m:024:   error: determinism declaration not satisfied.
a01.m:024:   Declared `det', inferred `semidet'.
a01.m:024:   The reasons for the difference are the following.
a01.m:025:   The switch on HeadVar__1 can fail.
a01.m:025:   In argument 2 of clause head:
a01.m:025:   in argument 2 of functor `[|]/2':
a01.m:025:   in argument 2 of functor `[|]/2':
a01.m:025:   unification with `list.[]' can fail.
a01.m:025:   In argument 2 of clause head:
a01.m:025:   in argument 2 of functor `[|]/2':
a01.m:025:   unification with `list.[V_23 | V_22]' can fail.
a01.m:025:   In argument 2 of clause head:
a01.m:025:   unification of `HeadVar__2' and `list.[V_21 | V_20]' can fail.

how do i make sure that these functions that use a pattern match are
det? Adding a branch with ; for the error cases makes it multi, but I
want to know how i can make it return a single value for every set of
arguments.