Is there a way to handle the first lapply element differently? or prepend it to the lapply result?

Is there a way to handle the first lapply element differently? or prepend it to the lapply result on the subsequent elements?

For example, I have the following:

mapping <- c("a", "b", "c")
# this doesn't matter here
#names(mapping) <- c("aa", "bb", "cc")
xmlvec <- c('<item a="1" c="2" />',
            '<item b="3" c="4" />',
            '<item a="5" b="6" c="7"/>')


df <- as.data.frame(do.call(
  rbind,
  lapply(xml_children(read_xml(paste("<xml>", paste(xmlvec, collapse=""), "</xml>"))),
         function(x) {
           y <- xml_attrs(x)[mapping]
           if (any(is.na(names(y)))) {
             y <- y[-which(is.na(names(y)))]
           }
           y[setdiff(mapping, names(y))] <- NA
           y[order(factor(names(y), levels=mapping))]
         }
  )
), stringsAsFactors = FALSE)
df

I'd like the first lapply element to be handled in such a way and all the subsequent ones only like this:

df <- as.data.frame(do.call(
  rbind,
  lapply(xml_children(read_xml(paste("<xml>", paste(xmlvec, collapse=""), "</xml>"))),
         function(x) {
           xml_attrs(x)[mapping]
         }
  )
), stringsAsFactors = FALSE)
df

The expected output for both cases (doing the bigger processing first or for every element) must be:

     a   b  c
1    1 <NA> 2
2 <NA>    3 4
3    5    6 7