You're seeing just the macro
assert_start_of_stream, go back to Membrane.Testing.Assertions module for more information.
Link to this macro
assert_start_of_stream(pipeline, element_name, pad \\ :input, timeout \\ 2000)View Source (macro)
Membrane.Testing.Pipeline received or is going to receive start_of_stream
notification from the element with a name
element_name within the
specified in milliseconds.