```I haven't explained the reason of the error estimation in
Range#step for Float;

double n = (end - beg)/unit;
double err = (fabs(beg) + fabs(end) + fabs(end-beg)) / fabs(unit) * epsilon;

The reason is as follows. (including unicode characters)
This is based on the theory of the error propagation;
http://en.wikipedia.org/wiki/Propagation_of_uncertainty

If f(x,y,z) is given as a function of x, y, z,
¦¤f (the error of f) can be estimated as:

¦¤f^2 = |¢ßf/¢ßx|^2*¦¤x^2 + |¢ßf/¢ßy|^2*¦¤y^2 + |¢ßf/¢ßz|^2*¦¤z^2

This is a kind of `statistical' error.  Instead, `maximum' error
can be expressed as:

¦¤f = |¢ßf/¢ßx|*¦¤x + |¢ßf/¢ßy|*¦¤y + |¢ßf/¢ßz|*¦¤z

I considered the latter is enough for this case.
Now, the target function here is:

n = f(e,b,u) = (e-b)/u

The partial differentiations of f are:

¢ßf/¢ße = 1/u
¢ßf/¢ßb = -1/u
¢ßf/¢ßu = -(e-b)/u^2

The errors of floating point values are estimated as:

¦¤e = |e|*¦Å
¦¤b = |b|*¦Å
¦¤u = |u|*¦Å

Finally, the error is derived as:

¦¤n = |¢ßn/¢ße|*¦¤e + |¢ßn/¢ßb|*¦¤b + |¢ßn/¢ßu|*¦¤u
= |1/u|*|e|*¦Å + |1/u|*|b|*¦Å + |(e-b)/u^2|*|u|*¦Å
= (|e| + |b| + |e-b|)/|u|*¦Å

Masahiro Tanaka

```