%% These are really functions.
    feet_per_meter -> 3.28.
    meter_per_foot -> 0.3048.

%% arc trig predicates

my_arcsin(A,X) :-
    sin(A) = X.

my_arccos(A,X) :-
    cos(A) = X.

my_arctan(A,X) :-
    tan(A) = X.

my_arctan2(A,Y,X) :-
    X >= 0.0,
    tan(A) = Y / X.

my_arctan2(A,Y,X) :-
    X < 0.0,
    Y >= 0.0,
    my_arcsin(PiD2,1,0),
    Pi = PiD2 * 2.0,
    tan(B) = Y / X,
    Pi + B = A.

my_arctan2(A,Y,X) :-
    X < 0.0,
    Y < 0.0,
    my_arcsin(PiD2,1,0),
    Pi = PiD2 * 2.0,
    tan(B) =  Y / X,
    -Pi + B = A.