Matrix differential equations not working

[seadra]

(initially reported in #797, which was prematurely closed)

There doesn't seem to be a way of defining matrix differential equation.

The code below,

@parameters x y
@variables u[1:2,1:2]
Dx = Differential(x)
A =  [1.0*x 2.0*y; 3.0 4.0];
eq  = Dx.(u) .~ A*u

B =  [1.0 2.0; 3.0 4.0];

bcs = [u(0,y) .~ 0.f0*B; u(1,y) .~ -sin(pi*1)*sin(pi*y)*B; u(x,0) .~ 0.f0*B; u(x,1) .~ -sin(pi*x)*sin(pi*1)*B]

which was partially based on suggestions in #797, fails with the following error:

MethodError: objects of type Array{Variable{Number},2} are not callable
Use square brackets [] for indexing an Array.

Stacktrace:
 [1] top-level scope at In[51]:6

[AayushSabharwal]

This works:

julia> @parameters x y
julia> @variables u(..)[1:2,1:2]
julia> Dx = Differential(x)
julia> A =  [1.0*x 2.0*y; 3.0 4.0];
julia> eq  = Dx(u(x, y)) ~ A*u(x, y)
julia> bcs = [u(0,y) ~ 0.f0*B; u(1,y) ~ -sin(pi*1)*sin(pi*y)*B; u(x,0) ~ 0.f0*B; u(x,1) ~ -sin(pi*x)*sin(pi*1)*B]
4-element Vector{Equation}:
 u(0, y) ~ [0.0 0.0; 0.0 0.0]
 u(1, y) ~ Num[-1.2246467991473532e-16sin(πy) -2.4492935982947064e-16sin(πy); -3.6739403974420594e-16sin(πy) -4.898587196589413e-16sin(πy)]
 u(x, 0) ~ [0.0 0.0; 0.0 0.0]
 u(x, 1) ~ Num[-1.2246467991473532e-16sin(πx) -2.4492935982947064e-16sin(πx); -3.6739403974420594e-16sin(πx) -4.898587196589413e-16sin(πx)]