'Open' - undeclared identifier

Does somebody know why this code inside a .mqh file throws the error 'Open' - undeclared identifier?

It seems like Open, Close, High, Low functions aren´t "detected" in my library. ( Other system functions like Print() are properly loaded ).

bool isBlueCandle( int candle ) export {
   return Open[candle] < Close[candle];
}

Not exactly, neither 1:1 copy, nor any MODs, return any error:

//+------------------------------------------------------------------+
//|  isBlueCandle TESTs                     MetaLang.exe: Build 1154 |
//+------------------------------------------------------------------+
bool isBlueCANDLE_TEST(        int candle ) export
{
   return Open[candle] < Close[candle];
}

bool isBlueCANDLE_TEST2(       int candle ) export {
   return Open[candle] < Close[candle];
}

bool isBlueCANDLE_TEST3( const int candle ) export {
   return Open[candle] < Close[candle];
}

bool isBlueCANDLE_TEST4( const int candle ) export {
   return( Open[candle] < Close[candle] );
}

As posted in the comment above, the missing context would help trace the root-cause for your stated issue.

Post a complete copy of the MetaLang.exe Error-description. Use mouse-right-click + copy ( in MetaLang.exe-Toolbox window on [Error]-page + paste that complete description on Stack Overflow )

As an example:

return value of 'OrderModify' should be checked
FOREX_SimpleSAR_EA_msMOD_0.00.mq4   227 19

Just for clarity

MQL4 recognises both functions ( Print() ) and other objects ( Open ) with specific access-protocol to work with them. In case of functions, one passes "arguments" compatible with the function's expectations.

Open, High, Volume et al, are not functions, but Arrays, the more, these arrays are special and carefully constructed in the internal MT4-engine, so as to provide a very fast & very efficient manipulation.

MetaQuotes call this a TimeSeries-object, a reversed-stepping-index into ( otherwise normal ) array.

So, your function isBlueCandle() is indeed a function, however, internally it does not call a function, but it compares a cell-values of Open ( the [anIntIndexAsPtrIntoTimeSeriesOrderedARRAY]-referenced cell ) against a value of Close ( namely the [anIntIndexAsPtrIntoTimeSeriesOrderedARRAY]-referenced cell ) to construct a bool which the isBlueCandle() function is about to return.