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The latest 13F season is here, when money managers issue reports on their holdings. It can be worthwhile to pay attention, as you might get an investment idea or two by seeing what some major investors have been buying and selling.

Tale famous value investor David Einhorn and his hedge fund Greenlight Capital. Einhorn's investing success, as well as his advocacy of financial transparency and accountability, has won him many fans. Although he isn't afraid to short stocks, Einhorn prefers going long with underpriced stocks. He started Greenlight with less than $1 million, and it now boasts a stock portfolio worth $7.7 billion.

Interesting developments
So what does Greenlight Capital's latest quarterly 13F filing tell us? Here are a few interesting details.

The biggest of a few new holdings is General Motors Company (NYSE:GM). General Motors has been accelerating its performance lately: Its recent quarterly earnings more than quadrupled from their year-ago levels. It has recently faced some problems, such as unfavorable currency translations because of the strong dollar, soft sales abroad, and an inability to keep up with demand at home. But that last one is a good problem to have, and CEO Mary Barra is bullish on GM's prospects as she works to drive up profit margins: "We have a very well-articulated plan. We're in the middle of the executing that, and we're not going to entertain anything that distracts us from accomplishing that." For patient believers, the company offers a 4.1% dividend yield and plans to spend $5 billion repurchasing shares between now and the end of 2016.

Greenlight Capital upped its stakes in CONSOL Energy (NYSE:CNX) and Chicago Bridge & Iron Company N.V. (NYSE:CBI) by 55% and 129%, respectively. CONSOL is Greenlight's fourth-largest holding, while Chicago Bridge & Iron comes in sixth.

CONSOL specializes in natural gas and coal. Its stock has slid some 29% over the past year, making it more attractive to believers. In its last quarter, the company posted record natural gas production, up 48% from the year-ago quarter, with management projecting 30% growth for 2015 and 20% for 2016. Its coal production costs fell significantly, too, though the price of coal dropped as well.

Chicago Bridge & Iron builds energy plants and other infrastructure. The company has been constrained as low energy prices have reduced its customers' abilities to spend. A recent potential growth-driver is news that the company might be tapped for building a liquefied natural gas plant in Mozambique -- one with a potential total price tag near $15 billion. In its last quarter, the company's earnings topped expectations and rose 39% from the prior-year quarter, with revenue rising 7%. That suggests greater efficiency and cost cuts.

Greenlight's top holding is Apple (NASDAQ:AAPL), but it's worth noting that it pared its position by 13%. There are many reasons to be bullish about Apple, as its iPhones are selling like ... well, iPhones. Last quarter, iPhone revenue surged 55% year over year on sales of more than 61 million phones. Apple's history of innovation also bodes well. But there are some concerns. Apple is heavily reliant on the iPhone, which generates about two-thirds of the company's revenue. The iPhone's rapid growth can't last forever, especially as more of the world's markets become saturated with smartphones. Not all of Apple's innovations will be home runs, and it remains to be seen how widely adopted the new Apple Watch will be.

Greenlight sold out of its stake in Covidien entirely during the quarter. The company has been acquired by medical device giant Medtronic PLC (NYSE:MDT) in a deal valued at more than $42 billion.

We should never blindly copy any investor's moves, no matter how talented the investor. But it can be useful to keep an eye on what smart folks are doing, and 13F forms can be great places to find intriguing candidates for our portfolios.