Relational matching (join + project back on left)

SYNOPSIS

    #(signature)

DESCRIPTION

This operator restricts its LEFT operand to tuples for which there exists 
at least one tuple in RIGHT that (naturally) joins. This is a shortcut 
operator for the following longer expression:

    (project (join xxx, yyy), [xxx's attributes])

EXAMPLE

    # Which suppliers supply at least one part?
    !(alf matching suppliers supplies)