let s = "0123456789"
let n = 3
let m = 2
let c = '3'
let z = "345"

// A: starting from n characters in and of m length;
print(s.Substring(n, m))
// B: starting from n characters in, up to the end of the string;
print(s[n..])
// C: whole string minus the last character;
print(s[..-1])
// D: starting from a known character within the string and of m length;
print(s.Substring(s.IndexOf(c),m))
// E: starting from a known substring within the string and of m length.
print(s.Substring(s.IndexOf(z),m))