Command Language

  [Commands]      simple-command
                  command-lookup-order
                  ysh-prefix-binding
                  semicolon ;
  [Redirects]     ysh-here-str    read <<< '''
  [YSH Simple]    typed-arg       json write (x)
                  lazy-expr-arg   assert [42 === x]
                  block-arg       cd /tmp { echo $PWD }; cd /tmp (; ; blockexpr)
  [YSH Cond]      ysh-case        case (x) { *.py { echo 'python' } }
                  ysh-if          if (x > 0) { echo }
  [YSH Iter]      ysh-for         for i, item in (mylist) { echo }
                  ysh-while       while (x > 0) { echo }
