uncertainty in measuring

this function computes the uncertainty in measurements.

suppose we have function of z(x,y). this program computes how the error in x and y will influence the error in z(x,y).
error in z we defined as
delta(z)=diff(z,x)*delta(x)+diff(z,y)*delta(y), where delta(x) and delta(y) the errors in x and y measurements.
diff(z,y) is the partial derivatives of z under y.