vars = @variables x, y

julia> Separator(vars, x)
ERROR: BoundsError: attempt to access 0-element Array{Expression,1} at index [1]
Stacktrace:
 [1] getindex at ./array.jl:729 [inlined]
 [2] parse_comparison(::Operation) at /Users/dpsanders/.julia/dev/IntervalConstraintProgramming/src/separator.jl:94
 [3] Separator(::Tuple{Operation,Operation}, ::Operation) at /Users/dpsanders/.julia/dev/IntervalConstraintProgramming/src/separator.jl:199
 [4] top-level scope at none:0

julia> Separator(vars, x^2 + x)
ERROR: MethodError: no method matching iterate(::Nothing)
Closest candidates are:
  iterate(::Core.SimpleVector) at essentials.jl:568
  iterate(::Core.SimpleVector, ::Any) at essentials.jl:568
  iterate(::ExponentialBackOff) at error.jl:199
  ...
Stacktrace:
 [1] indexed_iterate(::Nothing, ::Int64) at ./tuple.jl:66
 [2] Separator(::Tuple{Operation,Operation}, ::Operation) at /Users/dpsanders/.julia/dev/IntervalConstraintProgramming/src/separator.jl:199
 [3] top-level scope at none:0