Using the example from here...

```
#this works fine
f <- function(x, y, z) x^2+y^2+z^2
#but I want to try it with vectors it fails
vec_dot <- function(a, b) sum(a*t(b))
f <- function(x, y, z) vec_dot(c(x,y,z), c(x,y,z))
#check some values each function returns
for(y in 1:10) {print(f(y/10,0,0))}
#draw the isosurface
d <- seq(-2, 2, len=20)
contour3d(f,Tc,d,d,d,engine="standard")
```

When I run this I get:

```
Error in tris[3 * (1:n) - 2, ] : subscript out of bounds
Calls: contour3d ... contourTriangles -> makeTriangles -> unzipTriangleMatrix
```

But commenting out the second `f`

works, giving me a sphere. Both functions `f`

give the exact same values, as far as I can tell. Is there some hidden number-type-that-fails-with-contour3d that `sum()`

returns but `print()`

s exactly the same as a regular number?