Develop high-level recursive functions to accomplish each of the following,

Sum. The sum of the first n natural numbers.

Factorial. The product of first `n` natural numbers (written as `n!`). Special Case: `0! =1`

Fibonacci. `t_1=t_2=1; t_n=t_{n-1}+t_{n-2}`

Pascal's Triangle. `p_{r,0}=p_{r,r}=1; p_{r,c}=p_{r-1,c-1}+p_{r-1,c}`