lhs = rhs evaluates rhs and assigns the result to be the value of lhs. From then on, lhs is replaced by rhs whenever it appears. {l_1, l_2, ...} = {r_1, r_2, ...} evaluates ...

BitSet[n, k] sets to 1 the bit corresponding to the coefficient of 2^k in the integer n.

SetAttributes[s, attr] adds attr to the list of attributes of the symbol s.

SetOptions[s, name_1 -> value_1, name_2 -> value_2, ...] sets the specified default options for a symbol s. SetOptions[stream, ...] or SetOptions["name", ...] sets options ...

SetSelectedNotebook[obj] makes the notebook corresponding to obj be the currently selected one in the front end.

f /: lhs = rhs assigns rhs to be the value of lhs, and associates the assignment with the symbol f.

SetSystemOptions["name" -> value] resets the value for the internal system option with the specified name.

SetStreamPosition[stream, n] sets the current point in an open stream.

Mathematica usually keeps the elements of a list in exactly the order you originally entered them. If you want to treat a Mathematica list like a mathematical set, however, ...

SetAccuracy[expr, a] yields a version of expr in which all numbers have been set to have accuracy a.