LyX does not correctly import yex files

[Ehtibar Dzhafarov]

LyX 1.6.1 for Mac cannot correctly import tex files, even if the tex  
file has been previously exported from a LyX 1.6.1 file. When the tex  
file is imported, LyX does not recognize the environments (theorems,  
lemmas, definitions, etc.), leaves them as plain tex commands, and  
cannot compile the file (the error dialogue saying "unknown  
environment"). Version 1.5.7 has no problems of this sort.