A control flow statement modifies a program's [[w c:code:control flow|control flow]]. A control structure additionally contains another statement which is executed under specified conditions, by modification and/or validation of the environment. Furthermore, loops are structures which repeat their statements while the environment is validated by a given test, or "condition." (And a loop which does not modify its environment at some point in a way which ultimately makes its condition false is an [[w c:code:infinite loop|infinite loop]], a common type of bug.)