1.

s1 <- list(s='abc')
class(s1) <- 'string'

length.string <- function(a) nchar(a$s)
print.string <- function(a) print(a$s)

'+.string' <- function(a,b)
{  
   if (class(b) == 'string') b <- b$s
   tmp <- list(s=paste0(a$s,b))
   class(tmp) <- 'string'
   tmp
}

s2 <-  list(s='de')
class(s2) <- 'string'
length(s2)
s3 <- 'f'
s1+s2+s3
# "abcdef"
s1+s2+'uv'+s3
# "abcdeuvf"
s1+'uv'+s2
# "abcuvde"

2.

plot2densities <- function(x1,x2)
{
   d1 <- density(x1)
   d2 <- density(x2)
   d1taller <- max(d1$y) > max(d2$y)
   if (d1taller) {
      plot(d1)
      lines(d2)
   } else {
      plot(d2)
      lines(d1)
   }

}