FLtabs is the "file card tabs" interface that allows useful to display several areas containing widgets in the same windows, alternatively. It must be used together with FLgroup, another container that groups child widgets.
iwidth -- width of widget.
iheight -- height of widget.
ix -- horizontal position of upper left corner of the valuator, relative to the upper left corner of corresponding window. Expressed in pixels.
iy -- vertical position of upper left corner of the valuator, relative to the upper left corner of corresponding window. Expressed in pixels.
Containers are useful to format the graphic appearance of the widgets. The most important container is FLpanel, that actually creates a window. It can be filled with other containers and/or valuators or other kinds of widgets.
There are no k-rate arguments in containers.
FLtabs is a "file card tabs" interface that is useful to display several alternate areas containing widgets in the same window.

FLtabs.
It must be used together with FLgroup, another FLTK container opcode that groups child widgets.
The following example code:
FLpanel "Panel1",450,550,100,100
FLscroll 450,550,0,0
FLtabs 400,550, 5,5
FLgroup "sliders",380,500, 10,40,1
gk1,ihs FLslider "FLslider 1", 500, 1000, 2 ,1, -1, 300,15, 20,50
gk2,ihs FLslider "FLslider 2", 300, 5000, 2 ,3, -1, 300,15, 20,100
gk3,ihs FLslider "FLslider 3", 350, 1000, 2 ,5, -1, 300,15, 20,150
gk4,ihs FLslider "FLslider 4", 250, 5000, 1 ,11, -1, 300,30, 20,200
gk5,ihs FLslider "FLslider 5", 220, 8000, 2 ,1, -1, 300,15, 20,250
gk6,ihs FLslider "FLslider 6", 1, 5000, 1 ,13, -1, 300,15, 20,300
gk7,ihs FLslider "FLslider 7", 870, 5000, 1 ,15, -1, 300,30, 20,350
gk8,ihs FLslider "FLslider 8", 20, 20000, 2 ,6, -1, 30,400, 350,50
FLgroupEnd
FLgroup "rollers",380,500, 10,30,2
gk1,ihr FLroller "FLroller 1", 50, 1000,.1,2 ,1 ,-1, 200,22, 20,50
gk2,ihr FLroller "FLroller 2", 80, 5000,1,2 ,1 ,-1, 200,22, 20,100
gk3,ihr FLroller "FLroller 3", 50, 1000,.1,2 ,1 ,-1, 200,22, 20,150
gk4,ihr FLroller "FLroller 4", 80, 5000,1,2 ,1 ,-1, 200,22, 20,200
gk5,ihr FLroller "FLroller 5", 50, 1000,.1,2 ,1 ,-1, 200,22, 20,250
gk6,ihr FLroller "FLroller 6", 80, 5000,1,2 ,1 ,-1, 200,22, 20,300
gk7,ihr FLroller "FLroller 7",50, 5000,1,1 ,2 ,-1, 30,300, 280,50
FLgroupEnd
FLgroup "joysticks",380,500, 10,40,3
gk1,gk2,ihj1,ihj2 FLjoy "FLjoy", 50, 18000, 50, 18000,2,2,-1,-1,300,300,30,60
FLgroupEnd
FLtabsEnd
FLscrollEnd
FLpanelEnd

FLtabs example, sliders tab.

FLtabs example, rollers tab.

FLtabs example, joysticks tab.
FLgroup, FLgroupEnd, FLpack, FLpackEnd, FLpanel, FLpanelEnd, FLscroll, FLscrollEnd, FLtabsEnd