Operations with big integers

All operations are made with strings, you do not need do create any objects.
Where argument type is String, it also works with Number
There 14 functions:
add(String a, String b): returns String a+b, you can also call String.add(b) and it will return a+b without changing the original String
sub(String a, String b): returns String a-b, or String.sub(b), as above
mul(String a, String b): returns String a*b, or String.mul(b), as above
div(String a, String b): returns String Euclidean division, or String.div(b), as above
mod(String a, String b): returns String Euclidean division remainder, or String.mod(b), as above
pow(String a, Number b): returns String a^b, or String.pow(b), as above
sign(String n): returns Number (1, 0 ot -1), or String.sign(), as above
abs(String n): returns String abs(n), or String.abs(), as above
min(String a, String b): returns String which is less than another
max(String a, String b): returns String which is bigger than another
isbigger(String a, String b): returns Boolean is a>=b or not
sqr(String n): returns String a^2, or String.sqr(), as above
fibo(Number n): returns String n-th Fibonacci number
fact(Number n): returns String factorial of n

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Usage examples

	add(25, 30); //returns '55'
	sub('36', 11); //returns '25'
	"45".add(7); //returns '52'
	let n = '16';
	n.pow(3).add('10'); //returns '4106', 16^3 + 10