ASX edges lower; AGL sags after $2.7b writedown

Summary

• The ASX 200 dropped by as much as 0.4% to 6800.5, with Rio Tinto, CSL, Woolworths, and Amcor weighing 
• Wall Street rose for a third straight session overnight, with Google parent Alphabet jumping following its strong quarterly results
• Bitcoin added about 4% to $US37,500, and fellow cryptocurrency Ethereum rose more than 7% to a record $US1654
• Energy giant AGL will recognise a $2.69 billion charge in its first-half results, reflecting asset impairments and onerous contracts. Shares in the firm dropped 6% to a more than 12-year low at the open