For a full reference, see the Lemur User Guide available here. The appendixes at the end are particularly concise and informative.
This will assign 3.14 as the value of the Fader.foo expression. It's up to you to make sure the 'foo' expression exists, the parser will not throw an error because the setexpression() function performs the search only at run-time.
This will assign 5.0 as the value of the 'foo' expression in the object where the script is being run.
Warning: setexpression() has one limitation, it can't access global expressions in the root of the project (listed immediately at the top, under Project in the project hierarchy). To be accessed with setexpression(), expressions must be created within an object.
OSC messages with no arguments: It is not possible to send or receive an OSC message without any arguments. (such as “/asdf”). The Lemur script engine does not support 'nil' values to specify the absence of an argument. An array always contains at least one value in Lemur-land.