List and feature structure predicates

Step includes useful predicates for working with lists, and feature structures.

Lists

• [Member element list]
  True when element is an element of list.
• [Length list length]
  True when list has exactly length elements.
• [Nth list index element]
  True when the element of list at position index is element.  Note that programming languages traditionally number elements starting from 0.  So the first element is index 0, the second is index 1, etc.

Feature structures

• [HasFeature featureStructure feature]
  True when featureStructure contains a feature named feature.