This works great (thanks to Sunanda's suggestion http://stackoverflow.com/questions/1452239/rebol-how-to-parse-a-b-syntax-error/1453205#1453205):
attribute: copy []
class: copy []
definition-rule: compose/deep [some [set class word! 'is 'defined 'by
[some [copy attribute to (to-lit-word "/") thru (to-lit-word "/") ]]
copy attribute to end]]
parse [Customer is defined by First Name / Last Name / Email] definition-rule
but I now need to add some append instructions (append output class) and it doesn't work any more:
attribute: copy []
class: copy []
definition-rule: compose/deep [some [set class word! (append output class) 'is 'defined 'by
[some [copy attribute to (to-lit-word "/") thru (to-lit-word "/") ]]
copy attribute to end]]
parse [Customer is defined by First Name / Last Name / Email] definition-rule