code: def poisson(x: TensorType, lam: TensorType) -> tf.Tensor:
      return x * tf.math.log(lam) - lam - tf.math.lgamma(x + 1.0)
  
file_path: GPflow/gpflow/logdensities.py
imports: import numpy
  import tensorflow
  from check_shapes import check_shapes
  from .base import TensorType
  from .utilities import to_default_float
label_desc: Use image node_id 4 for calling a global function with example usage: poisson(x, lam) and returns: unknown