Builtin Commands

  [Memory]        append                 Add elements to end of array
                  pp                     asdl   cell   X gc-stats   line   proc
  [Handle Errors] try                    Run with errexit, set _status _error
                  boolstatus             Enforce 0 or 1 exit status
                  error                  error 'failed' (status=2)
  [Shell State]   ysh-cd   ysh-shopt     compatible, and takes a block
                  shvar                  Temporary modify global settings
                  ctx                    Share and update a temporary "context"
                  push-registers         Save registers like $?, PIPESTATUS
  [Modules]       runproc                Run a proc; use as main entry point
                  module                 guard against duplicate 'source'
                  is-main                false when sourcing a file
                  use                    change first word lookup
  [I/O]           ysh-read               flags --all, -0
                  ysh-echo               no -e -n with simple_echo
                  write                  Like echo, with --, --sep, --end
                  fork   forkwait        Replace & and (), and takes a block
                  fopen                  Open multiple streams, takes a block
                  X dbg                  Only thing that can be used in funcs
                  X log   X die          common functions (polyfill)
  [Hay Config]    hay   haynode          For DSLs and config files
  [Completion]    compadjust   compexport
  [Data Formats]  json                   read write
                  json8                  read write
                  X packle               read write, Graph-shaped
X [TSV8]          rows                   pick rows; dplyr filter()
                  cols                   pick columns ('select' already taken)
                  group-by               add a column with a group ID [ext]
                  sort-by                sort by columns; dplyr arrange() [ext]
                  summary                count, sum, histogram, etc. [ext]
  [Args Parser]   parser                 Parse command line arguments
                  flag
                  arg
                  rest
                  parseArgs()
X [Testing]       describe               Test harness
                  assert                 takes an expression
X [External Lang] BEGIN   END   when (awk)
                  rule (make)   each (xargs)   fs (find)
