public interface TaskInputExpression
  A TaskInputExpression is a String prefixed with "$input", "$inputRecord", "$last",
  or "$ruleScope" followed by an optional dot-separated hierarchical path, which can
  specify either an atomic data value (String, Number) or Record from the input data.
 For example, if the Process.state
 represented in JSON were:
  
  {
     orderId:5,
     orderItems: [
        {name:"Pencils", quantity:3, itemId:2344}
     ],
     orderUser: { name:"Henry Winkle", address:"...", ... }
  }
  
  .. and a task specified an inputField of "orderId" and an inputFieldList of
  "orderItems","orderUser", then:
  
  The other two sources of input are "$last" and "$ruleScope". The former references the
 contents of the transient state.
 Finally, "$ruleScope" can be used
 to pull values from a ruleScope when
 configured in Process.ruleScope.
  
execute() or, for an asynchronous task,
 the value passed to setOutputRecord() or 
      setOutputData().