^ is not equivalent to * for positive integer exponents in zero.

[rcalxrc08]

using TaylorDiff
x=TaylorScalar(0.0,1.0)
y_2=x^2
@assert isnan(TaylorDiff.extract_derivative(y_2,2))
y_1=x^1
@assert isnan(TaylorDiff.extract_derivative(y_1,2))

This doesn't happen in case of *

x=TaylorScalar(0.0,1.0)
y_2=x*x
@assert !isnan(TaylorDiff.extract_derivative(y_2,2))
y_4=x*x*x*x
@assert !isnan(TaylorDiff.extract_derivative(y_4,2))

[rcalxrc08]

I am sorry this is a duplicate of #33