<｜begin▁of▁sentence｜>### Instruction:
Given a tree graph created using `igraph.Graph.Tree(127, 2)`, how would you use the `DescartesCoordinateSystem.bbox` property in igraph to set a custom bounding box before plotting and saving the graph to 'graph_with_custom_bbox.png'?
### Response:
from igraph.drawing import plot
from igraph import Graph
from igraph.drawing.coord import DescartesCoordinateSystem

# Create an igraph graph
g = Graph.Tree(127, 2)

# Create a DescartesCoordinateSystem instance
cs = DescartesCoordinateSystem(g)

# Create a Plot object with the custom coordinate system
p = plot(g, bbox=(0, 0, 300, 300), vertex_size=10, vertex_label=None, coord_system=cs)

# Now, let's set the bounding box of the coordinate system
cs.bbox = (100, 100, 200, 200)

# Save the plot to a file
p.save('graph_with_custom_bbox.png')
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