Regarding Global Variable in QTP

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Code:

set objExcel=createobject("excel.application")
environment.value("wbk")=objExcel.workbooks.open("somepath")

now i wnt to use this wbk object in my function libaray

public function a()
  set objworkbooks=environment.value("wbk")
  set objworksheets=objworkbooks.sheets
end function

but it show some error objworkbooks dnt hve sheets method