Spherical 4-bar linkage

@Preben Alsholm 

Thank you very much. More with your help I found for myself now obvious way to remove an element from array (Array).

@Kitonum 

a := [x+1, x+2, x+3, x+4]:

remove(i -> i=x+2, a);

numelems(a); nops(a); op(2, a);

@Preben Alsholm 

restart;
a := [x+1, x+2, x+3, x+4];
numelems(a);
nops(a);
subs(x+2 = NULL, a);
nops(a); op(2, a);
a;
a := subs(x+2 = NULL, a);
nops(a);
op(2, a);

@Markiyan Hirnyk 

Very interesting, do you know the definition of parallel curves on surface in differential geometry? And yet you probably know the definition of a spiral on the surface?

https://www.google.ru/?gws_rd=ssl#newwindow=1&q=What+are+parallel+curves+on+surface

Parallel curves on the surface (1/6) * x1 ^ 2 + x2 ^ 2 + 10 * x 3 * 4-1 = 0

https://vk.com/doc242471809_320829803
https://vk.com/doc242471809_321212555 

        Initial curve on the surface:

x3-0.1e-1*exp(x1)/(0.1e-1+x1^4+x2^4)=0,

obtained from the intersection of the sphere:

x1^2+(x2-.5)^2+x3^2-2=0.

      Building a geodesic between selected points. Testing shows that the normal to the surface in any point of the our curve lies on the osculating plane. (Show a picture of all the points together with the planes is not possible, so one pointis shown in two positions and two corresponding osculating planes. In the initial positionof the osculating planeis painted in blue, in the end position – green.)
      Not every geodesic curve is the shortest distance between the points, but we can construct a set of geodetic and select from them the shortest. If an initial curve will be on the other side "bulge",  the first geodesic would be the shortest geodesic.

The same algorithm can be used to construct  parallel curves  on surfaces.
For example, three  parallel curves  on the surface:

(x1-sin((x1^2+x2^2+x3^2)^.5)^2)^2+(x2-sin(x1)^2)^2+(x3-sin(x1)^2)^2-9=0;

.gif file,    4-bar linkage spherical mechanism animation

@Carl Love 

Many examples on my page. Examples constructing geodesic repeated from the beginning (intermittently) to reduce file size and mechanisms try to finish smoothly animation to look smooth and continuous.
I understand the question? I just really do not know much English

@Carl Love 

Yes, it is. GIF. Usually inserted - through the green arrow ... More do not know anything. I have all the files .Gif animation as shown.

http://www.maplesoft.com/applications/view.aspx?SID=34940

and:

https://vk.com/doc242471809_332342637

https://vk.com/doc242471809_332344020

restart:
f:=(x2^20*x4^2)/(x1^10*x3^5*x5*x6^5);
n:=nops(f); 
x := seq(eval(cat('x', i)), i = 1 .. n);
for i to n do
k:= degree(f,x[i]); if k<0 then print( 1/x[i],k); fi ;od:

restart:

f := x1^2/(x2^3*x3^2*x4*x5^10);

n := 5;

x := seq(eval(cat('x', i)), i = 1 .. n);

for i from 2 to n do

op(1, op(i, f))^sign(op(2, op(i, f)))

                                                      end do;