module fostr

imports
  
  Common

context-free start-symbols
  
  Ex

context-free sorts

  Ex

context-free syntax
  
  Ex.Int      = INT
  Ex.Stdio    = <stdio>
  Ex.Sum      = {Ex "+"}+
  Ex.Receives = [[Ex] << [Ex]] {left}
  Ex.Enters   = [[Ex] >> [Ex]] {left}
  Ex          = <(<Ex>)> {bracket}

context-free priorities

  Ex.Enters
  > Ex.Sum
  > Ex.Receives,

  // prevent cycle: no singletons
  Ex.Sum <0> .> {Ex "+"}+ = Ex,

  // flat: no Sum immediately in Sum:
  {Ex "+"}+ = Ex <0> .> Ex.Sum,
  {Ex "+"}+ = {Ex "+"}+ "+" Ex <2> .> Ex.Sum