Find the value of an option, given a corresponding option name. This function provides an easy way to find the value of a particular option parser. If an option has multiple names, any one of them can be used as the argument to this function without changing the result.
If the option was not present, returns #f.
| parser | An instance of <argument-list-parser>. |
| long-name | An instance of <string>. |
| value | An instance of <object> or #f. |