R help - Mar 2014 - field of two possible types in reference classes

Dear R users,
I am learning the OOP programming with R, in particular with reference classes.

If I have a class, e.g.

myclass <- setRefClass(class="myclass",
fields=list(x="numeric"));

I know that if I do not want to set any class for x, I can just type
fields=list(x="ANY"). But what if
I want to allow x to be of two possible classes, e.g. "numeric" or
"logical". They do not inherit from each other, so setting the one and
passing the other class results in an error.

Any suggestions will be appreciated.

Best regards,

Martin