Why does an object named first_time appear in the global environment when I show GRangesList objects?

I tried to debug this to figure out where it gets created, but all I managed to figure out is that it shows up inexplicably while inside GenomicRanges:::.GenomicRanges_summary.

I see one mention on the support site in this post: A: Strange first_time object in rtracklayer import. However, this object is created even while running in a vanilla session of R in the terminal.

Here is my (hopefully) reproducible example:

gr  <- GenomicRanges::GRanges(
  seqnames = "chr2", 
  ranges   = IRanges::IRanges(3, 6), 
  strand   = "+")​

grl <- GenomicRanges::split(gr, "chr2")​ # creates a GRangesList object

ls()
## [1]  "gr"   "grl"

grl   # Show/print the object

ls()
## [1] "first_time"  "gr"  "grl"

sessionInfo()
version 3.3.2 (2016-10-31)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: macOS Sierra 10.12.2

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] zlibbioc_1.20.0      IRanges_2.8.1        XVector_0.14.0      
[4] parallel_3.3.2       GenomicRanges_1.26.1 S4Vectors_0.12.1    
[7] BiocGenerics_0.20.0  GenomeInfoDb_1.10.1  stats4_3.3.2

This comes from BiocGenerics, in particular the unsafe_replaceSlots function. The code is pretty well commented, so I will point you there for a rationale.