import { BigNumber } from "https://deno.land/x/web3@v0.11.1/types/bignumber.d.ts";
Returns a BigNumber whose value is the value of this BigNumber plus n.
The return value is always exact and unrounded.
0.1 + 0.2 // 0.30000000000000004
x = new BigNumber(0.1)
y = x.plus(0.2) // '0.3'
BigNumber(0.7).plus(x).plus(y) // '1.1'
x.plus('0.1', 8) // '0.225'