def all_variables

      return nil if @expressions.empty?

      @expressions.each_with_object({}) { |exp, h|
        h[exp.fei] = exp.variables if exp.variables
      }
    end
def all_variables

      return nil if @expressions.empty?

      @expressions.each_with_object({}) { |exp, h|
        h[exp.fei] = exp.variables if exp.variables
      }
    end
Returns a hash fei => variable_hash containing all the variable bindings
 (expression by expression) of the process instance.