def input(step_property, &value)
      mapping = IOMapping.new
      mapping.property = step_property
      mapping.value = value
      @step.inputs << mapping
      self
    end
def input(step_property, &value)
      mapping = IOMapping.new
      mapping.property = step_property
      mapping.value = value
      @step.inputs << mapping
      self
    end
Map workflow instance data to a property on the step

 @param step_property [Symbol] the attribute on the step body class