[plt-scheme] define-struct and auto fields

hi,

what i think i get from the docs is that there's only one auto value
per struct, and that value is used to init any auto fields no mattery
how many different auto fields there might be. i don't understand why
i can't specify an auto value for each auto field. like, what if one
is supposed to hold a number, and another is supposed to hold a list?
(is plt's implementation of structs very different from that of other
schemes?)

thanks.