[Mrql Wiki] Trivial Update of "LanguageDescription" by LeonidasFegaras

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The "LanguageDescription" page has been changed by LeonidasFegaras:
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  result of the expression e' under these bindings. For
  example, {{{let x = 1 in x+1}}}.
  
- == Accessing the Data Sources ==
+ == Accessing Data Sources ==
  
  The MRQL expression that makes a directory of raw files accessible
  to a query is: 
@@ -1144, +1144 @@

  centroid calculated at every step are stored in memory (they are not
  a persistent bag). 
  
+ == Tracing Query Execution ==
+ 
+ Any expression {{{e}}} in a query can be traced during query execution using {{{trace(e)}}}. This will print the value of the expression every time it is evaluated. Nested trace calls will appear as nested traces.
+ 
  == System Functions ==
  
  [[SystemFunctions|The MRQL System functions]]