Predicates Within Strings

[Justin Choi via Digitalmars-d-learn]

Could somebody explain or point me to documentation that helps to 
explain the usage of strings in predicates?
My main question is how D infers the omitted variable 
specifications given otherwise - for example:
`filter!(a => a < 3)(arr);`
and
`filter!"a < 3"(arr);`
produce the same result.