Find a char among string (string.findAmong.char)

[BoQsc via Digitalmars-d-learn]

I get an error when I try to find that letter is among alphabet.
onlineapp.d(13): Error: template 
`std.algorithm.searching.findAmong` cannot deduce function from 
argument types `!()(immutable(char), immutable(string))`, 
candidates are:
/dlang/dmd/linux/bin64/../../src/phobos/std/algorithm/searching.d(2694):        
`findAmong(alias pred = "a == b", InputRange, ForwardRange)(InputRange seq, 
ForwardRange choices)`
  with `pred = "a == b",
       InputRange = immutable(char),
       ForwardRange = string`
  must satisfy the following constraint:
`       isInputRange!InputRange`


This is the code: You can run it at: 
https://run.dlang.io/is/5cvuUZ
 import std;
 void main()
 {
     alias alphabet = letters;
     char[26] letters = ['a','b', 'c', 'd', 'e',
                         'f', 'g', 'h', 'i', 'j',
                         'k', 'l', 'm', 'n', 'o',
                         'p', 'q', 'r', 's', 't',
                         'u', 'v', 'w', 'x', 'y', 'z'];

     string wordExample = "Book.";
     foreach (letter; wordExample){
         if (letter.findAmong(alphabet)){
             write("found");
         }
        write(letter);
     }
                                                
 }