PositionConstraint
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Definition::

    # PositionConstraint message base_navigation
    
    string  frame
    string  constraint
    
    # frame: the coordinate frame in which the position constraint is active
    #
    #  - examples:  "map", "person" , "table"
    
    # constraint: constraint expression that defines the desired position
    #
    #  - variables: x, y
    #
    #  - example:   "x > 5 && x < 10 && y > 5 && y < 10"
    #               "x^2 + y^2 < 2^2"
    #
    #  - info:      Mathimatical expression: * / ^ + - and all other operators that are begin supported by: exprtk - http://www.partow.net/programming/exprtk/index.html