neg(a) = islowercase(a) ? uppercase(a) : lowercase(a)

function sample(atoms)
    result = Char[]
    for a in atoms
        if rand(Bool)
            push!(result, rand(Bool) ? a : neg(a))
        end
    end
    return length(result) > 0 ? join(result) : "λ"
end

function sample(n::Int, atoms)
    result = String[]
    for _ in 1:n
        push!(result, sample(atoms))
    end
    return result
end

using DelimitedFiles

abc_atoms = [ 'a', 'b', 'c']

open("sample.csv", "w") do io
    writedlm(io, [ "event" ])
    writedlm(io, sample(100, abc_atoms), )
end