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That's works perfectly, thanks!
Out of curiosity then, why did it work when I compiled to tex source, outside of Lyx?

Addendum: It seems to have something to do with the ams package. using any ams symbol directly (no macro) anywhere in the document, allows for compilation. This includes actually calling \leqslant, followed by \leq (with \leq --> \leqslant).

Hi,

thanks for the quick reply. Here's the log file for compiling without \subsetneq.

Hello,

I seem to be experiencing an odd issue with lyx.

I have a macro mapping \leq to \leqslant.
Withing the document I'm editing this works fine, but if I start a new document in which the only macro is the one above and the only symbol is \leq, I get an undefined control sequence error. There ...