How to overload a function with arguments of class sym?

How can I create an overloaded function of type sym, for use by the symbolic tool box?

I can do this for double and char, by creating the functions in @double and @char subdirectories, but this doesn't seem to work for @sym. For example:

@char/myfunc.m:

function p = myfunc(x)
    p = 'char arg';
end

@double/myfunc.m:

function p = myfunc(x)
    p = 'double arg';
end

@sym/myfunc.m:

function p = myfunc(x)
    p = 'sym arg';
end

Running which -all seems to find it:

>> which -all myfunc()

C:\Users\me\MATLAB\@char\myfunc.m % char method

C:\Users\me\MATLAB\@double\myfunc.m % double method

C:\Users\me\MATLAB\@sym\myfunc.m % sym method

But not when called with the corresponding argument:

>>which myfunc(1234)

C:\Users\me\MATLAB\@double\myfunc.m % double method

>> which myfunc('1234')

C:\Users\me\MATLAB\@char\myfunc.m % char method

>> which myfunc(sym(0))

'myfunc(sym(0))' not found.

>>class(sym(0))

ans =

'sym'

Any ideas? Thanks.