Experiment with these parsers

*MyParser> let p = satisfy isUpper in parse p ""
Nothing
*MyParser> let p = satisfy isUpper in parse p "abc"
Nothing
*MyParser> let p = satisfy isUpper in parse p "Abc"
Just ("bc",'A')
*MyParser> let p = is 'a' in parse p ""
Nothing
*MyParser> let p = is 'a' in parse p "abc"
Just ("bc",'a')
*MyParser> let p = is 'a' in parse p "xbc"
Nothing