Aldehydes Ketones And Carboxylic Acids

The major product of the following reaction is :


$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}=\mathrm{O} \xrightarrow[\text { reflux }]{\substack{\text { excess } \mathrm{HCHO} \\ \text { alkali }}}$ ?