%% \ifDefined symbol
%%
%% If the symbol is defined, returns its music expression value.
%% Otherwise, returns a void expression.

ifDefined =
#(define-music-function (parser location sym) (symbol?)
  (let ((music (ly:parser-lookup parser sym)))
   (if (ly:music? music)
    music
    (make-music 'Music 'void #t))))

%% \ifDefinedElse symbol expression
%%
%% If the symbol is defined, returns its music expression value.
%% Otherwise, returns the expression.

ifDefinedElse =
#(define-music-function (parser location sym else) (symbol? ly:music?)
  (let ((music (ly:parser-lookup parser sym)))
   (if (ly:music? music) music else)))

%% \ifDefinedThen symbol expression
%%
%% If the symbol is defined, returns the expression.
%% Otherwise, returns a void expression.

ifDefinedThen =
#(define-music-function (parser location sym music) (symbol? ly:music?)
  (if (defined? sym)
   music
   (make-music 'Music 'void #t)))

%% \ifDefinedThenElse symbol expression else-expression
%%
%% If the symbol is defined, returns the expression.
%% Otherwise, returns the else-expression.

ifDefinedThenElse =
#(define-music-function (parser location sym music else) (symbol? ly:music? ly:music?)
  (if (defined? sym)
   music else))

%% Example:
%%
%% \ifDefinedThen #'leadWords  \lyricsto lead \ifDefined #'leadWords