PDF
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Definition::

    uint8 EXACT=0     # Is here for easier msg creation, but is actually a specialized case:
                      # - continuous: is a Gaussian with zero covariance
                      # - discrete: is a Discrete pdf with one entry with P=1                
    uint8 GAUSSIAN=1
    uint8 UNIFORM=2
    uint8 PARTICLES=3
    uint8 MIXTURE=4
    uint8 DISCRETE=5
    uint8 HYBRID=6
    
    uint8 type
    uint32 dimensions
    
    # For continuous pdf's
    float64[] data
    
    # For discrete pdf's (pmf)
    int32 domain_size
    string[] values
    float64[] probabilities
    
    # For exact values, either a string or a vector
    string exact_value_str
    float64[] exact_value_vec