_i_n_O_p_c_o_d_e Specifies the callback function variable.
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
ggggllllccccGGGGeeeettttCCCCaaaallllllllbbbbaaaacccckkkkFFFFuuuunnnncccc returns the value of the callback function variable
specified by _i_n_O_p_c_o_d_e. Currently, _i_n_O_p_c_o_d_e can only have the value
GGGGLLLLCCCC____OOOOPPPP____ggggllllccccUUUUnnnnmmmmaaaappppppppeeeeddddCCCCooooddddeeee. The initial value of that variable is GGGGLLLLCCCC____NNNNOOOONNNNEEEE
and the type signature is GGGGLLLLbbbboooooooolllleeeeaaaannnn((((****))))((((GGGGLLLLiiiinnnntttt iiiinnnnCCCCooooddddeeee)))).
EEEERRRRRRRROOOORRRRSSSS
GGGGLLLLCCCC____SSSSTTTTAAAATTTTEEEE____EEEERRRRRRRROOOORRRR is generated if the issuing thread has no current GLC