Debugger entered--Lisp error: (wrong-type-argument integerp (+ -10))
  set-frame-position(#<frame emacs@xxx 0xyyyyyyy> (+ -10) 14)
<略>

(modify-frame-parameters nil '((left . 50)))
(modify-frame-parameters nil '((left . -50)))
(modify-frame-parameters nil '((left + -50)))
(modify-frame-parameters nil '((left - -50)))

(x-parse-geometry "80x25+50+0")
=>
((height . 25) (width . 80) (top . 0) (left . 50))

(x-parse-geometry "80x25-50+0")
=>
((height . 25) (width . 80) (top . 0) (left . -50))

(x-parse-geometry "80x25+-50+0")
=>
((height . 25) (width . 80) (top . 0) (left + -50))

(x-parse-geometry "80x25--50+0")
=>
((height . 25) (width . 80) (top . 0) (left - -50))