add optional parameter to stackrel to put something below the relation symbol, compatible with salign

lecture.cls

@@ -25,6 +25,7 @@
 \RequirePackage{bookmark}
 \RequirePackage{wasysym}
 \RequirePackage{environ}
+\RequirePackage{stackrel}
 
 \usetikzlibrary{quotes, angles}
 
@@ -134,8 +135,8 @@
 
 % better stackrel
 \let\oldstackrel\stackrel
-\renewcommand{\stackrel}[2]{%
-    \oldstackrel{\mathclap{#1}}{#2}
+\renewcommand{\stackrel}[3][]{%
+    \oldstackrel[\mathclap{#1}]{\mathclap{#2}}{#3}
 }%
 
 % integral d sign
@@ -183,6 +184,7 @@
 \box_new:N \l_tmp_box
 \dim_new:N \l_tmp_dim_a
 \dim_new:N \l_tmp_dim_b
+\dim_new:N \l_tmp_dim_c
 \dim_new:N \l_tmp_dim_needed
 
 % function to insert padding according to widest stackrel
@@ -190,7 +192,7 @@
  {
   \tl_set:Nn \l__lec_text_tl { #1 }
   % get all stackrels in this align environment
-  \regex_extract_all:nnN { \c{stackrel}{(.*?)}{(.*?)} } { #1 } \l_lec_stackrels_seq
+  \regex_extract_all:nnN { \c{stackrel}(\[.*?\])?{(.*?)}{(.*?)} } { #1 } \l_lec_stackrels_seq
   % get number of stackrels
   \int_set:Nn \l_stackrel_count_int { \seq_count:N \l_lec_stackrels_seq }
   \int_set:Nn \l_idx_int { 1 }
@@ -201,30 +203,28 @@
       % calculate width of text
       \hbox_set:Nn \l_tmp_box {$\seq_item:Nn \l_lec_stackrels_seq { \l_idx_int + 1 }$}
       \dim_set:Nn \l_tmp_dim_a {\box_wd:N \l_tmp_box}
-      % calculate width of relation symbol
       \hbox_set:Nn \l_tmp_box {$\seq_item:Nn \l_lec_stackrels_seq { \l_idx_int + 2 }$}
+      \dim_set:Nn \l_tmp_dim_c {\box_wd:N \l_tmp_box}
+      \dim_set:Nn \l_tmp_dim_a {\dim_max:nn{ \l_tmp_dim_c} {\l_tmp_dim_a}}
+      % calculate width of relation symbol
+      \hbox_set:Nn \l_tmp_box {$\seq_item:Nn \l_lec_stackrels_seq { \l_idx_int + 3 }$}
       \dim_set:Nn \l_tmp_dim_b {\box_wd:N \l_tmp_box}
       % check if 0.5*(a-b) > minimum padding, if yes updated minimum padding
       \dim_compare:nNnTF
         { 1pt * \dim_ratio:nn { \l_tmp_dim_a - \l_tmp_dim_b } { 2pt } } > { \l_tmp_dim_needed }
         { \dim_set:Nn \l_tmp_dim_needed { 1pt * \dim_ratio:nn { \l_tmp_dim_a - \l_tmp_dim_b } { 2pt } } }
         { }
-      \quad
       % increment list index by three, as every stackrel produces three list entries
       \int_incr:N \l_idx_int
       \int_incr:N \l_idx_int
       \int_incr:N \l_idx_int
+      \int_incr:N \l_idx_int
   }
   % replace all relations with align characters (&) and add the needed padding
   \regex_replace_all:nnN
-  { (\c{iff}&|&\c{iff}|\c{impliedby}&|&\c{impliedby}|\c{implies}&|&\c{implies}|\c{approx}&|&\c{approx}|\c{equiv}&|&\c{equiv}|=&|&=|\c{le}&|&\c{le}|\c{ge}&|&\c{ge}|&\c{stackrel}{.*?}{.*?}|\c{stackrel}{.*?}{.*?}&|&\c{neq}|\c{neq}&|>&|&>|<&|&<) }
+  { (\c{iff}&|&\c{iff}|\c{impliedby}&|&\c{impliedby}|\c{implies}&|&\c{implies}|\c{approx}&|&\c{approx}|\c{equiv}&|&\c{equiv}|=&|&=|\c{le}&|&\c{le}|\c{ge}&|&\c{ge}|&\c{stackrel}(\[.*?\])?{.*?}{.*?}|\c{stackrel}(\[.*?\])?{.*?}{.*?}&|&\c{neq}|\c{neq}&|>&|&>|<&|&<) }
       { \c{kern} \u{l_tmp_dim_needed} \1 \c{kern} \u{l_tmp_dim_needed} }
       \l__lec_text_tl
-  % replace all parantheses with \left( \right)
-  %\regex_replace_all:nnN { \( } { \c{left}( } \l__lec_text_tl
-  %\regex_replace_all:nnN { \) } { \c{right}) } \l__lec_text_tl
-  %\regex_replace_all:nnN { \[ } { \c{left}[ } \l__lec_text_tl
-  %\regex_replace_all:nnN { \] } { \c{right}] } \l__lec_text_tl
   \l__lec_text_tl
  }
 \cs_generate_variant:Nn \lec_insert_padding:n { V }