function installed($formula)
    {
        return in_array($formula, explode(PHP_EOL, $this->cli->runAsUser('brew list | grep '.$formula)));
    }
function installed($formula)
    {
        return in_array($formula, explode(PHP_EOL, $this->cli->runAsUser('brew list | grep '.$formula)));
    }
function installed ( $ formula ) { return in_array ( $ formula , explode ( PHP_EOL , $ this -> cli -> runAsUser ( 'brew list | grep ' . $ formula ) ) ) ; }
Determine if the given formula is installed.

@param  string  $formula
@return bool
Determine if the given formula is installed .