>> To compute a tangent to an arbitrary curve defined as f(t) (the value of 
>> which
>> in this case is a vector), you need the derivative f'(t) of that 
>> function,
>
>  I find those two sentences contradictory. First you say that it's
> impossible, and immediately after you tell how it's done.

Really? Can a macro in POV SDL calculate the exact mathematical derivative 
of a function?  I didn't know that.

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