[users@httpd] proxy for www and without

[Marc]

How should I change this config so it works with www.a.com and a.com?  I tried 
with trying to assign %{HTTP_HOST} to defaulthost


    Define defaulthost "a.com"
    Define proxyhost "b.com"

    ProxyPreserveHost Off
    ProxyAddHeaders On
    SetOutputFilter proxy-html
    ProxyHTMLEnable On
    ProxyHTMLExtended On

    ProxyPass        "https://${proxyhost}/xxx";
    ProxyPassReverse "https://${proxyhost}/xxx";

    ProxyPassReverseCookieDomain "${proxyhost}" "${defaulthost}"
        
    ProxyHTMLURLMap https://${proxyhost}/xxx https://${defaulthost}/en/yyyy