In 4.01 CS01, we introduced the ability to insert a filter into a path using a special "filter path segment" of the form

Customers/$filter=@q/myns.action?$@q=Age gt 18

(see https://issues.oasis-open.org/browse/ODATA-836)

It turns out, the facility to append parameter information to a segment was considered over 20 years ago by Tim Berners-Lee using "matrix parameters" (http://www.w3.org/DesignIssues/MatrixURIs.html) – basically, a matrix parameter is a way to add parameters to a path segment as a series of key-value pairs separated from the path segment with a semi-colon.

4.01 CS01 syntax:
Customers/$filter=@q?$@q=name eq 'Smith'
Matrix parameter syntax:
Customers;$filter=@q?$@q=name eq 'Smith'

Advantages of using matrix parameters:
1) No possible ambiguity with key-as-segment
2) Not inventing something new
3) May already be supported by some systems