Spin coefficients for four-dimensional neutral metrics, and null   geometry
  Notation for spin coefficients for metrics of neutral signature in four dimensions is introduced. The utility and interpretation of spin coefficients is explored through themes in null geometry familiar from (complex) general relativity. Four-dimensional Walker geometry is exploited to provide examples and the generalization of the real neutral version of Pleba\~nski's (1975) second heavenly equation to certain Walker geometries given in Law and Matsushita [16] is extended further. 
Peter R. Law