<jupyter_start><jupyter_text>WorldThe world element encapsulates an entire world description including: models, scene, physics, joints, and plugins.<jupyter_code># Import the element creator
from pcg_gazebo.parsers.sdf import create_sdf_element
# The default world contains only the definition of the acceleration of gravity 
# and the default configuration of the physics engine using `ode`
world = create_sdf_element('world')
print(world)
# The physics engine can be edited using its object setter
world.physics.reset(mode='ode', with_optional_elements=True)
print(world)
world.physics.reset(mode='bullet', with_optional_elements=True)
print(world)
world.physics.reset(mode='simbody', with_optional_elements=True)
print(world)
# A world can contain multiple models, add an empty model by using the add_model method
world.reset()
world.add_model('model_1')
print(world)
# You can add a model to the world as well
model = create_sdf_element('model')
world.add_model('model_2', model)
print(world)
# You must have unique names for the models in a world
world.add_model('model_1')<jupyter_output>Model element with name model_1 already exists
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