Experiment with the choice parser

*MyParser> let p = character ||| value 'x' in parse p "abc"
Just ("bc",'a')
*MyParser> let p = character ||| value 'x' in parse p ""
Just ("",'x')
*MyParser> let p = character ||| failed in parse p "abc"
Just ("bc",'a')
*MyParser> let p = character ||| failed in parse p ""
Nothing