simple lesson wanted for overloading list

[Andrew Jonathan Fine]

Hi,

Suppose I have the following abstract type:

:: Greek = Alpha | Beta | Gamma | Delta | Epsilon

I want to do

Start :: Bool
Start = isMember Alpha [Alpha Beta Gamma]

but I get the error that isMember has no instance

How to I overload the StdList operators for an abstact type?

Thanks,

Andrew Jonathan Fine