Simple filter expression is also an array. It contains one or more filter terms, combined with operators - 'AND', 'OR', 'NOT'. (Operators are case insensitive):
[ 
  ['mainline', 'is', 'T'],
  'and', ['type','anyof',['CustInvc','CustCred']],
  'and', 'not', ['amount', 'equalto', '0.00'],
  'or'...