Confusing SFG error message on missed connection

ValueError: Unconnected input port in SFG is going to be confusing error message for students during the lab session once the eventually miss a connection in a SFG.

MRE:

##
# SFG of Lab1: Task 2
#
from b_asic.core_operations import Addition, ConstantMultiplication
from b_asic.special_operations import Input, Output, Delay
from b_asic.signal_flow_graph import SFG

x = Input()
y = Output()
d1, d2 = (Delay(), Delay())
b1, b2, a0, a1, a2 = (
    ConstantMultiplication( 179/512, d1), # b1
    ConstantMultiplication(-171/512, d2), # b2
    ConstantMultiplication(  57/256),     # a0
    ConstantMultiplication(  55/128, d1), # a1
    ConstantMultiplication(  57/256, d2), # a2
)
add0 = Addition(b1, b2)
add1 = Addition(a1, a2)
add2 = Addition(x, add0)
add3 = Addition(a0, add1)
# a0.input(0).connect(add2)  # <--- Forgotten input
d1.input(0).connect(add2)
d2.input(0).connect(d1)
y.input(0).connect(add3)
sfg = SFG([x], [y])
sfg

runcell(2, '/home/mikhe33/education/tste87/2024/lab1/lab1.py')
Traceback (most recent call last):

  File ~/education/tste87/2024/basic-venv/lib/python3.11/site-packages/spyder_kernels/py3compat.py:356 in compat_exec
    exec(code, globals, locals)

  File ~/education/tste87/2024/lab1/lab1.py:72
    sfg = SFG([x], [y])

  File ~/education/tste87/2024/B-ASIC/b_asic/signal_flow_graph.py:262 in __init__
    self._add_operation_connected_tree_copy(

  File ~/education/tste87/2024/B-ASIC/b_asic/signal_flow_graph.py:1276 in _add_operation_connected_tree_copy
    raise ValueError("Unconnected input port in SFG")

ValueError: Unconnected input port in SFG