problem while importing a set of integro-differential equations

edit

Ok, the error was

'sage.rings.integer.Integer' object is not callable

when I was calling the operators using expressions like -- IntR2(phi[j][n](x1=x2,y1=y2)*g(j,phi[i][n])) -- but then I redefined the operators such that themselves include the -- (x1=x2,y1=y2) -- expressions instead of the operator caller! So that now the error became as

ValueError: Computation failed since Maxima requested additional constraints; using the 'assume' command before integral evaluation *may* help (example of legal syntax is 'assume(t>0)', see `assume?` for more details)
Is  t  positive, negative, or zero?

This is while I still use the dummy variables td, xd1, and xd2, and of course the assume(t>0) statement! Any idea what to do now?

My minimal code now is this:

reset()
forget()
var('x,y,t, x1,y1, x2,y2')
var('x_B1,x_B2, y_B1,y_B2, T')

x_B1=-1; x_B2=1; y_B1=-1; y_B2=1; T=10
assume(x_B1<=x, 0<t)

var('q')                # q is a dummy variable to fill the zeroth place in some lists

R0=[q,x,y,t]
R1=[x1,y1]
R2=[x2,y2]

phi=[q]
for i in range(1,4):
    phi.append([])

    for n in range(3):
        phi[i].append(function('phi_%s_%s' %(i,n), x,y,t,*R1+R2))



# initial estimation

for i in range(1,4):
    phi[i][0]=1



# operators
var('td,xd1,xd2')
g = lambda i,f: diff(f,R0[i])
It = lambda f: integral(f(t=td),td,0,t)
Ixx = lambda f: integral(integral(f(x=xd1),xd1,x_B1,xd2),xd2,x_B1,x)
IntR2 = lambda f:  integral(integral(f(x1=x2,y1=y2),x2,x_B1,x_B2),y2,y_B1,y_B2)



# equations being imported

for n in range(2):
    eq=[]
    for i in range(1,3):
        # FIRST. a minimal full-featured typical equation.
        # RESULT: the error was ('sage.rings.integer.Integer' object is not callable) when I was calling the operators using expressions like -- IntR2(phi[j][n](x1=x2,y1=y2)*g(j,phi[i][n])) -- but now that the operators are redefined to themselves include the -- (x1=x2,y1=y2) -- expressions the error became as (ValueError: Computation failed since Maxima requested additional constraints; using the 'assume' command before integral evaluation *may* help … Is  t  positive, negative, or zero?)
        eq.append(phi[i][n+1]== It(g(i,phi[i][n])) \
                                + It(  sum( phi[j][n]*g(j,phi[i][n]) \
                                + IntR2(phi[j][n]*g(j,phi[i][n])) for j in range(1,4) ) ))

    show(eq)

What shall I do now?

EDIT.

I have just confused. Right now I tested the following code and it surprisingly worked!

reset()
forget()

var('x,y,z,t, x1,y1,z1,t1, x2,y2,z2,t2, x3,y3,z3,t3') 
R0=[x,y,z,t]
R1=[x1,y1,z1,t1]

U_0=[]
U_1=[]


for i in range(4):
    U_0.append(function('U%s0' %i, *R0))
    U_1.append(function('U%s1' %i, *R0+R1))


var('td,xd1,xd2,x_B1')
It = lambda f: integral ...