String Abstraction
We abstract over strings and characters so that we do not rely on any specific encoding. We only need the following:
- a function to compute length of strings,
- a function to fetch a character at an index, and
- a function to check if two characters are equal.
length s should compute the length of the string s
get s i should fetch the character at index i of a string s
val equal : ch -> ch -> boolequal c1 c2 should return true if c1 and c2 are equal, and false otherwise.