abstract type Panel <: Solution end

struct Panel2D <: Panel
    xs :: Float64
    ys :: Float64
    xe :: Float64
    ye :: Float64
    xc :: Float64
    yc :: Float64
    length :: Float64
    angle :: Float64
    loc :: T where T <: AbstractString
    function Panel2D(xs, ys, xe, ye)
        xc, yc   = (xe + xs)/2.0, (ye + ys)/2.0
        length = mag([ xe - xs, ye - ys ])
        angle = xe <= xs ? acos((ye - ys)/length) : π + acos(-(ye - ys)/length)
        loc = angle <= π ? "upper" : "lower"
        new(xs, ys, xe, ye, xc, yc, length, angle, loc)
    end
end

mutable struct SourcePanel2D <: Panel2D
    panel :: Panel2D
    strength :: Float64
    vt :: Float64
    cp :: Float64
    function SourcePanel2D(panel, strength=0.0, vt=0.0, cp=0.0)
        new(panel, strength, vt, cp)
    end
end