Checks if a function is a S3 generic function.
# S3 method for default
isGenericS3(fcn, envir=parent.frame(), ...)If argument fcn is a character, this is the
environment from which the search for the function is done.
Not used.
Returns TRUE if a generic S3/UseMethod function, otherwise FALSE.
A function is considered to be a generic S3/UseMethod function if
its name matches one of the known S3 generic functions, or if it
calls UseMethod().