How can I define a Regexp::Grammar rule that ignores leading whitespaces?

Question

From the Regexp::Grammars documentation:

The difference between a token and a rule is that a token treats any whitespace within it exactly as a normal Perl regular expression would. That is, a sequence of whitespace in a token is ignored if the /x modifier is in effect, or else matches the same literal sequence of whitespace characters (if /x is not in effect).

In a rule, most sequences of whitespace are treated as matching the implicit subrule <.ws>, which is automatically predefined to match optional whitespace (i.e. \s*).

...

In other words, a rule such as:

<rule: sentence>   <noun> <verb>
               |   <verb> <noun>

is equivalent to a token with added non-capturing whitespace matching:

<token: sentence>  <.ws> <noun> <.ws> <verb>
                |  <.ws> <verb> <.ws> <noun>

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Is there a way to get the rule to ignore the leading implicit <.ws>? In the example above, it would be equivalent to:

<token: sentence>  <noun> <.ws> <verb>
                |  <verb> <.ws> <noun>