Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.70%. Comparing base (356c48a) to head (bfbf17c).
Report is 6 commits behind head on master.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   90.27%   90.70%   +0.43%     
==========================================
  Files          54       54              
  Lines        3055     3046       -9     
==========================================
+ Hits         2758     2763       +5     
+ Misses        297      283      -14     

Can you explain why this check idx !== nothing is needed?

function Base.getindex(m::Motion, p)
    idx, spin_range = m.spins[p]
    return idx !== nothing ? Motion(m.action[idx], m.time, spin_range) : nothing
end

Can you explain why this check idx !== nothing is needed?

function Base.getindex(m::Motion, p)
    idx, spin_range = m.spins[p]
    return idx !== nothing ? Motion(m.action[idx], m.time, spin_range) : nothing
end

I need this when, for example I have a m=Motion with SpinRange(501:1000) and I do m[1:500], that is, when indexing with out-of-range indices. As the intersection between the two ranges is null, the return SpinRange would be SpinRange(0:0), but that makes no sense. So, in that case, it is better to return nothing.
Imagine that this motion is part of a phantom obj with 1000 spins:

julia> obj.motion
MotionList(
   Translate(..., SpinRange(501:1000))
)

When indexing obj[1:500]: the resulting motion should be NoMotion

julia> obj[1:500].motion
NoMotion(...)

Ok! Feel free to merge.