### 2.21.17 Rewrite trigonometric functions as function of tan(x/2) and hyperbolic functions as function of exp(x):
`halftan_hyp2exp`

`halftan_hyp2exp` takes as argument a trigonometric and hyperbolic expression.
`halftan_hyp2exp` rewrites sin(x), cos(x), tan(x) in terms of tan(x/2) and sinh(x), cosh(x), tanh(x) in terms of exp(x).
Input :

`halftan_hyp2exp(tan(x)+tanh(x))`

Output :

`(2*tan(x/2))/((1-(tan(x/2))```^```2))+(((exp(x))```^```2-1))/ (((exp(x))```^```2+1))`

Input :

`halftan_hyp2exp(sin(x)```^```2+cos(x)```^```2-sinh(x)```^```2+cosh(x)```^```2)`

Output, after simplification with `normal(ans())` :

`2`