Where does the template parameter E come from?

[simendsjo]

When running compose with two arguments, the implementation uses the 
template parameter E. I don't understand what's going on here as E isn't 
submitted to the template - what type is E?
I've also seen this in unary/binaryFun using ElementType.

template compose(fun...) { alias composeImpl!(fun).doIt compose; }

// Implementation of compose
template composeImpl(fun...)
{
    static if (fun.length == 1)
    {
        static if (is(typeof(fun[0]) : string))
            alias unaryFun!(fun[0]) doIt;
        else
            alias fun[0] doIt;
    }
    else static if (fun.length == 2)
    {
        // starch
        static if (is(typeof(fun[0]) : string))
            alias unaryFun!(fun[0]) fun0;
        else
            alias fun[0] fun0;
        static if (is(typeof(fun[1]) : string))
            alias unaryFun!(fun[1]) fun1;
        else
            alias fun[1] fun1;
        // protein: the core composition operation
        typeof({ E a; return fun0(fun1(a)); }()) doIt(E)(E a)
        {
            return fun0(fun1(a));
        }
    }
    else
    {
        // protein: assembling operations
        alias composeImpl!(fun[0], composeImpl!(fun[1 .. $]).doIt).doIt 
doIt;
    }
}