The :keyword:`!for`...\ :keyword:`!in` clauses contain the sequences to be iterated over.The sequences do not have to be the same length, because they are *not* iterated over in parallel, but from left to right; this is explained more clearly in the following paragraphs.The elements of the generated list will be the successive values of *expression*.The final :keyword:`!if` clause is optional; if present, *expression* is only evaluated and added to the result if *condition* is true.