how to use Natural type?

[demotomohiro]

In mathematics, integer and natural number are infinite set. But `int` and 
`Natural` type in Nim (and most of programming languages) can store only finite 
subset of integer or natural number. So calling them as integer or natural 
number might looks misreading for people who have math background. [Ring of 
integers modulo 
n](https://en.wikipedia.org/wiki/Modular_arithmetic#Integers_modulo_n) or ℤ/nℤ 
might be more accurate name for them. But people would not like such a long or 
unicode name.