TASK DEFINITION: Generate a correct and concise answer for the question based on the words in the context.
PROBLEM: Context : OPML (Outline Processor Markup Language) is an XML format for outlines (defined as ``a tree, where each node contains a set of named attributes with string values''). 
Question : Which is the basis of OPML?

SOLUTION: XML

PROBLEM: Context : The Boeing 601 (sometimes referred to as the BSS-601, and previously as the HS-601) is a communications satellite bus introduced in 1987 by Hughes Space and Communications Company. 
Question : What year was Boeing 601 commissioned?

SOLUTION: 1987

PROBLEM: Context : Nicholas Barr FRSA is a British economist, currently serving as professor of public economics at the London School of Economics (LSE). 
Question : What was Nicholas Barr's occupation?

SOLUTION:
economist