R: How come that "is" is TRUE, but "as" is impossible?

Question

The class "ecdf" inherits from the class "stepfun". If f is such an empirical cumulative density function, both is.stepfun(f) and is(f,"stepfun") are TRUE, and as.stepfun(f) doesn't do anything as expected. But conversion of f to "stepfun" by as(f,"stepfun") is impossible because of the "metadata", even if strict is FALSE:

f <- ecdf(1:10)

class(f)
# [1] "ecdf"     "stepfun"  "function"

is.stepfun(f)
# [1] TRUE

is(f,"stepfun")
# [1] TRUE

identical(f,as.stepfun(f))
# [1] TRUE

g <- as(f,"stepfun",strict=FALSE)
# Error in as(f, "stepfun", strict = FALSE) : 
#   internal problem in as(): “ecdf” is(object, "stepfun") is TRUE, but the metadata asserts that the 'is' relation is FALSE

So how is is related to as and what is the meaning of the "metadata" here?

Answer 1

I may have found some relevant information. At this nabble archive

but it has two problems: 

1) as() is an S4 method that does not always work 
(problem 2 not relevant)

Locally :-) this SO question has warnings about trying to use as()

So my suggestion would be to stick with as.stepfun(foo) .