CONVERT and variables for arguments

[epp]

Too bad CONVERT doesn't support variables for arguments. Anyone have a code for a conversion utility?

Code:


EXPORT C2,C1;
EXPORT TEST()
BEGIN
    LOCAL y,lst,a1,a2;
    CHOOSE(C2, "Convert from", "mm","cm","m","km","inches","feet","yards","miles");
    CHOOSE(C1, "Convert to",   "mm","cm","m","km","inches","feet","yards","miles");
    lst := {"mm","cm","m","km","inch","ft","yd","mile"};
    a1 := "1_"+lst[C1];
    a2: = "1_"+lst[C2];
    INPUT(y);
    y*CONVERT(a2,a1);
END;

Update 26 Oct 2015:
My first PRIME program and, over the last week, it has evolved to this solution.