Courses allows you to tag source code blocks using a special syntax hiding in the comments of the host language (so far only Python is supported, but this will change soon). So far, the placeholder/solution syntax is the only fully implemented function and it makes it possible to define a single source for documents that contain elements that have to be hidden from the recipients.
This concept is planned to be expanded to support automatic testing of solutions and grading of user submitted code.
hello
print("hello")