Does style_apply "average" patterns during merge?

[Tammo Spalink <tspalink telemetry-investments com>]

I have some code (in a plugin function):

> Sheet *sheet = eval_sheet (ei->pos->sheet, ei->pos->sheet);
> GnmStyle *style = gnm_style_new ();
> GnmColor *color = style_color_new_name ("red");
> GnmRange range = ... ;
> gnm_style_set_pattern_color (style, color);
> gnm_style_set_pattern (style, GO_PATTERN_FOREGROUND_SOLID);
> sheet_style_apply_range (sheet, &range, style);

The (unexpected, at least to me) result is that cells in the range
have their pattern set to GO_PATTERN_THIN_DIAG_CROSS...

Is there some merging of style fields going on?  My range only spans
cells in a new unmolested sheet.  Also, the above code seems to match
the (outdated?) instructions in Styles.txt.  A preliminary look at the
code underlying sheet_style_apply_range looks like there is no
"averaging" going on...  My reading of gnm_style_merge indicates that
those fields set in my code should be propagated directly.

Any insight would be much appreciated.  Thanks,

Tammo