Match only things where the block evaluates to true.

Methods
Public Class methods
new(&block)
    # File lib/flexmock/argument_matchers.rb, line 45
45:     def initialize(&block)
46:       @block = block
47:     end
Public Instance methods
===(target)
    # File lib/flexmock/argument_matchers.rb, line 48
48:     def ===(target)
49:       @block.call(target)
50:     end
inspect()
    # File lib/flexmock/argument_matchers.rb, line 51
51:     def inspect
52:       "on{...}"
53:     end