如何在FiPy中写自变量方程项？

from fipy import CellVariable, Grid1D, DiffusionTerm, TransientTerm, Viewer

nx = 100  # 网格单元数
dx = 1.0  # 网格间距
mesh = Grid1D(nx=nx, dx=dx)

phi = CellVariable(name="phi", mesh=mesh, value=0.0)

D = 1.0  # 扩散系数
eq = TransientTerm() == DiffusionTerm(coeff=D) - phi

valueLeft = 1.0  # 左边界值
valueRight = 0.0  # 右边界值
phi.constrain(valueLeft, mesh.facesLeft)
phi.constrain(valueRight, mesh.facesRight)

timeStepDuration = 0.1  # 时间步长
steps = 100  # 迭代步数
viewer = Viewer(vars=phi)  # 创建一个查看器，用于可视化结果

for step in range(steps):
    eq.solve(var=phi, dt=timeStepDuration)
    viewer.plot()