Hell Racketeers,

I am trying to write an implementation of the mathematical concept of dual
quaternions in Racket. Dual quaternions are an algebraic type and there are
several equally valid way to look at them.

Let me give some background first. A dual number (a + bε) is similar to a
complex number (a + bi), except that the square of ε is 0 instead of -1. A
quaternion (a + bi + cj + dk) is like a complex number, except that it has
three complex units which all square to -1. A dual quaternion is either a 
number where both components are quaternions, or a quaternion where all four
components are dual numbers. There are also "subtypes" of dual quaternoins: 
dual scalar is a dual quaternions where the vector parts of the two 
are zero, and a dual vector (not to be confused with the dual vectors from
linear algebra) is a dual quaternions where the scalar part of both 
is zero. Unlike the subtypes in OOP these algebraic subtypes have *less*
information than their parent (kind of like a square is a rectangle defined 
only one length instead of two). (If you want to get really down to it, dual
quaternions are elements of the Clifford algebra Cl^+(0, 3, 1).)

So my question is: what would be the best way of defining such algebraic 
At first I wanted to create a struct:

    (define vector (i j k))
    (struct quaternion (scalar vector))
    (struct dual-quaternon (real dual))

But this limits me to a "dual number where both components are quaternions"
view of the type. It also means that each dual scalar has be a sort of
mishapped dual quaternion.

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