function returns the substring of the first argument starting at the position specified in the second argument with length specified in the third argument.
The substring-after function returns the substring of the first argument string that follows the first occurrence of the second argument string in the first argument string, or the empty string if the first argument string does not contain the second argument string.
function returns the substring of the first argument string that precedes the first
occurrence of the second argument string in the first argument string, or the empty string if the first argument string
does not contain the second argument string.