Transforming a continued fraction representation into a real

The dfc2f command transforms a continued fraction into a real number.

dfc2f takes L, a list representing a continued fraction, which can be:
- a list of integers for a rational number.
- a list whose last element is a list for an ultimately periodic representation, i.e. a quadratic number, that is a root of a second order equation with integer coefficients.
- a list with a remainder r as last element (a=a₀+1/…+1/a_n+1/r).
dfc2f(L) returns the rational number or the quadratic number whose continued fraction representation is L.

dfc2f([1,2,[2]])

√

After simplification with normal, we obtain √2.

dfc2f([1,2,3])

normal(dfc2f([3,3,6,[3,6]]))

√

dfc2f([1,2,3,4,5,6,7])

9976

6961

Input to verify:

1+1/(2+1/(3+1/(4+1/(5+1/(6+1/7)))))

9976

6961

dfc2f([1,2,3,4,5,43/7])

9976

6961

Input to verify:

1+1/(2+1/(3+1/(4+1/(5+7/43))))

9976

6961