Macro impl_variant_param 

macro_rules! impl_variant_param {
    ($ty:ty, $cat:expr) => { ... };
    ($ty:ty, $cat:expr, k: $k:expr) => { ... };
}

Implement VariantParam and optionally KValue for a type.

Usage

// Variant parameter:
impl_variant_param!(SimpleGraph, "graph");

// Generic K value:
impl_variant_param!(KN, "k", k: None);

// Concrete K value:
impl_variant_param!(K3, "k", k: Some(3));