anchor.diff:  |java.math.BigDecimal
   |the statistical variance for the population for all non-null values
   
  -//|agg(_expression_)
  -//|any
  -//|any
  -//|customizes the aggregation logic in _expression_ and returns single value
  +|agg(_expression_)
  +|any
  +|any
  +|customizes the aggregation logic in _expression_ and returns single value
   
   |===
   
  
positive.code: include::../test/org/apache/groovy/ginq/GinqTest.groovy[tags=ginq_aggfunction_02,indent=0]
  ----
  
  ////
  [source, groovy]
  ----
  include::../test/org/apache/groovy/ginq/GinqTest.groovy[tags=ginq_aggfunction_01,indent=0]
  ----
  ////
  
  [source, groovy]
  ----
  
negative.code: [NOTE]
  `_g` is an implicit variable for `agg` aggregate function,
  it represents the grouped `Queryable` object and its record(e.g. `r`) could reference the data source by alias(e.g. `n`)
  
  
  Also, we could apply the aggregate functions for the whole GINQ result, i.e. no `groupby` clause is needed:
  [source, groovy]
  