The options passed to C<has> define the properties of the
attribute. There are many options, but in the simplest form you just
need to set C<is>, which can be either C<rw> (read-write) or C<ro>
-(read-only). C<rw> permits mutation of the attribute's value using the
-accessor. C<ro> does not; you may only read the current value of the
-attribute.
+(read-only). When an attribute is C<rw>, you can change it by passing
+a value to its accessor. When an attribute is C<ro>, you may only read
+the current value of the attribute.
In fact, you could even omit C<is>, but that gives you an attribute
that has no accessor. This can be useful with other attribute options,