Macro filter 

macro_rules! filter {
    ($field:expr, $pred:expr) => { ... };
}

Simplified where_ macro that reduces boilerplate.

Example

// Instead of:
.where_(Product::price(), |&p| p < 100.0)

// Write:
.where_(Product::price(), |&p| p < 100.0)  // Same for now, helper in query! macro