YSH Command Language Keywords

  [Assignment]    const   var     Declare variables
                  setvar          setvar a[i] = 42
                  setglobal       setglobal d.key = 'foo'
  [Expression]    equal =         = 1 + 2*3
                  call            call mylist->append(42)
  [Definitions]   proc            proc p (s, ...rest) {
                                  typed proc p (; typed, ...rest; n=0; b) {
                  func            func f(x; opt1, opt2) { return (x + 1) }
                  ysh-return      return (myexpr)
